tests: posix: common: test pthread_getconcurrency()

Add functional tests for pthread_getconcurrency() and
pthread_setconcurrency().

Note: the specification explicitly says

> an implementation can always ignore any calls to
> pthread_setconcurrency() and return a constant for
> pthread_getconcurrency()

The implementation and tests could be up for revision at a future
time when optimizations are considered and there is a better
system in placeo for documenting POSIX options and deviations.

Any such optimizations should be explicitly controlled via
Kconfig.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>