tests/semaphore: fix "cpu test took too long" assertion failure

The SMP config for RISC-V on QEMU triggers this:

|START - test_sem_queue_mutual_exclusion
|
|Assertion failed at
| WEST_TOPDIR/zephyr/subsys/testsuite/ztest/src/ztest_new.c:155:
| cpu_hold: (dt < 3000 is false)
|1cpu test took too long (4090 ms)
|ERROR: cannot fail in test 'after()', bailing

Looping 10000 times is maybe a bit excessive.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>