tidy: Fix paths to coretests and alloctests

Following `#135937` and `#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 332fb7e6f1d (Move std::thread_local unit tests
  to integration tests, 2025-01-17) and b8ae372e483 (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 949c96660c3 (move format! interface
  tests, 2020-09-08).

0 files changed, 0 insertions, 0 deletions