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.

• Move run_concurrently() to test.support.threading_helper to resuse 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.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @colesbury for commit 269b454 🤖

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