tests: posix: semaphore: speed up named semaphore test

Reduce N_LOOPS from 999 to 32 by default and use a Kconfig
to encode it as CONFIG_TEST_SEM_N_LOOPS.

Running TESTSUITE semaphore
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START - test_named_semaphore
 PASS - test_named_semaphore in 5.685 seconds
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START - test_semaphore
 PASS - test_semaphore in 5.010 seconds
===============================================================
TESTSUITE semaphore succeeded

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