tests: posix: semaphores: ensure test is not skipped

[cfriedt]

Describe the bug
Previously, the POSIX_SEMAPHORES Option Group was lumped-in with the tests/posix/common testsuite. That testsuite had a bit of a "CI optimization" so as to not run the tests twice (once with dynamic threads enabled, and once without).

The testsuite was copied over, and so was the "hack" to optimize CI. However, without properly ensuring that dynamic threads were configured properly, all tests were skipped.

I think a good policy for some contributors would be to fill in a "Testing Done" section in pull-requests, to verify that tests are actually executing the way that one one would expect.

Please also mention any information which could help others to understand
the problem you're facing:

• What target platform are you using? any
• What have you tried to diagnose or workaround this issue? found the root cause and solution
• Is this a regression? Yes, f7633a5

To Reproduce

1. west build -p auto -b qemu_riscv64 -t run tests/posix/semaphores
2. See that tests are skipped

Expected behavior
All tests should run and succeed.

Impact
The impact of this bug is low, since tests were passing before and after the fix.

Logs and console output

*** Booting Zephyr OS build v4.1.0-2563-g90cd350c5a4a ***
Running TESTSUITE posix_semaphores
===================================================================
START - test_named_semaphore
 SKIP - test_named_semaphore in 0.001 seconds
===================================================================
START - test_semaphore
 SKIP - test_semaphore in 0.001 seconds
===================================================================
TESTSUITE posix_semaphores succeeded

------ TESTSUITE SUMMARY START ------

SUITE SKIP -   0.00% [posix_semaphores]: pass = 0, fail = 0, skip = 2, total = 2 duration = 0.002 seconds
 - SKIP - [posix_semaphores.test_named_semaphore] duration = 0.001 seconds
 - SKIP - [posix_semaphores.test_semaphore] duration = 0.001 seconds

------ TESTSUITE SUMMARY END ------

===================================================================
PROJECT EXECUTION SUCCESSFUL

Environment (please complete the following information):

• OS: Any
• Toolchain (e.g Zephyr SDK, ...): Zephyr SDK v0.17.0
• Commit SHA or Version used: 90cd350

Additional context
See also #88766