gh-116738: Make grp module thread-safe

[yoney]

Make grp module methods getgrgid() and getgrnam() thread-safe when the GIL is disabled and getgrgid_r()/getgrnam_r() C APIs are not available.

• Introduce Lib/test/test_free_threading/test_ft.py to reuse run_concurrently() across multiple tests.

cc: @mpage @colesbury @Yhg1s

• Issue: Audit all built-in modules for thread safety #116738

[yoney]

Note: I am not sure if there is a CI setup where getgrgid_r()/getgrnam_r() are not available. I tested those manually on my local machine.