tests: posix: common: split xsi_realtime_threads into standalone test

[cfriedt]

Is your enhancement proposal related to a problem? Please describe.
Make the xsi_realtime_threads-related tests in tests/posix/common into a standalone test.

Describe the solution you'd like
The obvious way to group xsi_realtime_threads-related testsuites is by XSI_REALTIME_THREADS (See #86061).

Describe alternatives you've considered
The way the testsuite has been designed historically.

Additional context
See #79454 for more info