tests: Reduce code duplication. * tests/atomic-int-posix.h: New file, extracted from tests/test-pthread-mutex.c. * tests/test-pthread-mutex.c: Include it. Remove the corresponding code. * tests/test-pthread-rwlock.c: Likewise. * modules/pthread-mutex-tests (Files): Add tests/atomic-int-posix.h. * modules/pthread-rwlock-tests (Files): Likewise.