tidy: Fix paths to coretests and alloctests

Following `rust-lang#135937` and `rust-lang#136642`, tests for core and alloc are in
coretests and alloctests. Fix tidy to lint for the new paths. Also,
update comments referring to the old locations.

Some context for changes which don't match that pattern:
* library/std/src/thread/local/dynamic_tests.rs and
  library/std/src/sync/mpsc/sync_tests.rs were moved under
  library/std/tests/ in 332fb7e (Move std::thread_local unit tests
  to integration tests, 2025-01-17) and b8ae372 (Move std::sync unit
  tests to integration tests, 2025-01-17), respectively, so are no
  longer special cases.
* There never was a library/core/tests/fmt.rs file. That comment
  previously referred to src/test/ui/ifmt.rs, which was folded into
  library/alloc/tests/fmt.rs in 949c966 (move format! interface
  tests, 2020-09-08).

Author: thaliaarchi

Diff for: library/core/src/fmt/mod.rs

@@ -3016,6 +3016,6 @@ impl<T: ?Sized> Debug for SyncUnsafeCell<T> {
     }
 }
 
-// If you expected tests to be here, look instead at the core/tests/fmt.rs file,
+// If you expected tests to be here, look instead at coretests/tests/fmt/;
 // it's a lot easier than creating all of the rt::Piece structures here.
-// There are also tests in the alloc crate, for those that need allocations.
+// There are also tests in alloctests/tests/fmt.rs, for those that need allocations.

Diff for: library/coretests/tests/str.rs

@@ -1 +1 @@
-// All `str` tests live in library/alloc/tests/str.rs
+// All `str` tests live in library/alloctests/tests/str.rs

Diff for: src/tools/tidy/src/unit_tests.rs

@@ -1,11 +1,13 @@
-//! Tidy check to ensure `#[test]` and `#[bench]` are not used directly inside `core`.
+//! Tidy check to ensure `#[test]` and `#[bench]` are not used directly inside
+//! `core` or `alloc`.
 //!
-//! `#![no_core]` libraries cannot be tested directly due to duplicating lang
-//! items. All tests and benchmarks must be written externally in `core/{tests,benches}`.
+//! `core` and `alloc` cannot be tested directly due to duplicating lang items.
+//! All tests and benchmarks must be written externally in
+//! `{coretests,alloctests}/{tests,benches}`.
 //!
-//! Outside of core tests and benchmarks should be outlined into separate files
-//! named `tests.rs` or `benches.rs`, or directories named `tests` or `benches` unconfigured
-//! during normal build.
+//! Outside of `core` and `alloc`, tests and benchmarks should be outlined into
+//! separate files named `tests.rs` or `benches.rs`, or directories named
+//! `tests` or `benches` unconfigured during normal build.
 
 use std::path::Path;
 
@@ -14,40 +16,51 @@ use crate::walk::{filter_dirs, walk};
 pub fn check(root_path: &Path, bad: &mut bool) {
     let core = root_path.join("core");
     let core_copy = core.clone();
-    let core_tests = core.join("tests");
-    let core_benches = core.join("benches");
-    let is_core = move |path: &Path| {
-        path.starts_with(&core)
-            && !(path.starts_with(&core_tests) || path.starts_with(&core_benches))
-    };
+    let is_core = move |path: &Path| path.starts_with(&core);
+    let alloc = root_path.join("alloc");
+    let alloc_copy = alloc.clone();
+    let is_alloc = move |path: &Path| path.starts_with(&alloc);
 
     let skip = move |path: &Path, is_dir| {
         let file_name = path.file_name().unwrap_or_default();
         if is_dir {
             filter_dirs(path)
                 || path.ends_with("src/doc")
-                || (file_name == "tests" || file_name == "benches") && !is_core(path)
+                || (file_name == "tests" || file_name == "benches")
+                    && !is_core(path)
+                    && !is_alloc(path)
         } else {
             let extension = path.extension().unwrap_or_default();
             extension != "rs"
-                || (file_name == "tests.rs" || file_name == "benches.rs") && !is_core(path)
-                // UI tests with different names
-                || path.ends_with("src/thread/local/dynamic_tests.rs")
-                || path.ends_with("src/sync/mpsc/sync_tests.rs")
+                || (file_name == "tests.rs" || file_name == "benches.rs")
+                    && !is_core(path)
+                    && !is_alloc(path)
+                // Tests which use non-public internals and, as such, need to
+                // have the types in the same crate as the tests themselves. See
+                // the comment in alloctests/lib.rs.
+                || path.ends_with("library/alloc/src/collections/btree/borrow/tests.rs")
+                || path.ends_with("library/alloc/src/collections/btree/map/tests.rs")
+                || path.ends_with("library/alloc/src/collections/btree/node/tests.rs")
+                || path.ends_with("library/alloc/src/collections/btree/set/tests.rs")
+                || path.ends_with("library/alloc/src/collections/linked_list/tests.rs")
+                || path.ends_with("library/alloc/src/collections/vec_deque/tests.rs")
+                || path.ends_with("library/alloc/src/raw_vec/tests.rs")
         }
     };
 
     walk(root_path, skip, &mut |entry, contents| {
         let path = entry.path();
         let is_core = path.starts_with(&core_copy);
+        let is_alloc = path.starts_with(&alloc_copy);
         for (i, line) in contents.lines().enumerate() {
             let line = line.trim();
             let is_test = || line.contains("#[test]") && !line.contains("`#[test]");
             let is_bench = || line.contains("#[bench]") && !line.contains("`#[bench]");
             if !line.starts_with("//") && (is_test() || is_bench()) {
                 let explanation = if is_core {
-                    "core unit tests and benchmarks must be placed into \
-                         `core/tests` or `core/benches`"
+                    "`core` unit tests and benchmarks must be placed into `coretests`"
+                } else if is_alloc {
+                    "`alloc` unit tests and benchmarks must be placed into `alloctests`"
                 } else {
                     "unit tests and benchmarks must be placed into \
                          separate files or directories named \