gh-127586: properly restore blocked signals in resource_tracker.py (GH-127587)

* Correct pthread_sigmask in resource_tracker to restore old signals

Using SIG_UNBLOCK to remove blocked "ignored signals" may accidentally
cause side effects if the calling parent already had said signals
blocked to begin with and did not intend to unblock them when
creating a pool. Use SIG_SETMASK instead with the previous mask of
blocked signals to restore the original blocked set.

* Adding resource_tracker blocked signals test

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>