tests: POSIX: Enhance semaphore tests

Enhance semaphore test to validate sem_timedwait()
    and sem_trywait(). This is intended to improve
    code coverage.

Also modify ztest check with zassert_equal with
return value instead of zassert_false for better
understandability.

Signed-off-by: Spoorthi K <spoorthi.k@intel.com>