bootstrap: use specific-purpose ui test path

I wanted to move some ui tests around, which broke `test_valid` since it
was referencing a non-specific-purpose ui test.

Author: jieyouxu

src/bootstrap/src/core/builder/tests.rs

@@ -91,7 +91,7 @@ macro_rules! rustc {
 #[test]
 fn test_valid() {
     // make sure multi suite paths are accepted
-    check_cli(["test", "tests/ui/attr-start.rs", "tests/ui/attr-shebang.rs"]);
+    check_cli(["test", "tests/ui/bootstrap/self-test/a.rs", "tests/ui/bootstrap/self-test/b.rs"]);
 }
 
 #[test]

tests/ui/bootstrap/self-test/a.rs

@@ -0,0 +1,2 @@
+//! Not used by compiler, this is used by bootstrap cli self-test.
+//@ ignore-test

tests/ui/bootstrap/self-test/b.rs

@@ -0,0 +1,2 @@
+//! Not used by compiler, used by bootstrap cli self-test.
+//@ ignore-test