Rollup merge of rust-lang#137679 - bjorn3:coretests_improvements, r=jieyouxu,onur-ozkan

Various coretests improvements

The first commit is not yet strictly necessary as directly testing libcore works though useless work, but will be necessary once rust-lang#136642 migrates the liballoc tests into a separate package. The second commit fixes rust-lang#137478 and ensures that coretests actually gets tested on all CI job. The third commit fixes an error that didn't get caught because coretests doesn't run on the wasm32 CI job.

Author: jieyouxu

Diff for: coretests/tests/slice.rs

@@ -5,8 +5,6 @@ use core::num::NonZero;
 use core::ops::{Range, RangeInclusive};
 use core::slice;
 
-use rand::seq::IndexedRandom;
-
 #[test]
 fn test_position() {
     let b = [1, 2, 3, 5, 5];
@@ -1810,6 +1808,7 @@ fn select_nth_unstable() {
     use core::cmp::Ordering::{Equal, Greater, Less};
 
     use rand::Rng;
+    use rand::seq::IndexedRandom;
 
     let mut rng = crate::test_rng();