An attempt to fix mypyc tests on MacOS (#16520)

Fixes #16420

Although this is not 100% clear yet, but after 20 runs on a Mac I have
it no longer fails (without this patch it failed 20% of times). Btw,
contrary to the comment, _my_ Linux Mint (which is an Ubuntu derivative)
works perfectly (i.e. test passed 20 times even after I removed the
`sleep()`). So it is not really Mac vs Linux issue.

mypyc/test/test_run.py

@@ -172,12 +172,10 @@ def run_case_inner(self, testcase: DataDrivenTestCase) -> None:
  # new by distutils, shift the mtime of all of the
  # generated artifacts back by a second.
  fudge_dir_mtimes(WORKDIR, -1)
- # On Ubuntu, changing the mtime doesn't work reliably. As
+ # On some OS, changing the mtime doesn't work reliably. As
  # a workaround, sleep.
- #
  # TODO: Figure out a better approach, since this slows down tests.
- if sys.platform == "linux":
- time.sleep(1.0)
+ time.sleep(1.0)
 
  step += 1
  with chdir_manager(".."):