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.

Author: ilevkivskyi

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(".."):