tests: posix: mqueue: increase sleep ms to reduce failures

The test_mqueue_notify_thread testcase had a relatively high
failure rate because sleeping only 10ms was in some cases not
sufficiently long enough for the spawned thread to update
notification_executed.

Sleep 100 ms instead of 10 ms to reduce the failure rate.

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