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

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.

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