tests: kernel/sys_mutex: sleep a bit for private_mutex to lock

With SMP, the private_mutex may not be locked fast enough by
thread_12 after the thread creation as it might take longer
for a thread to start running the entry function, resulting
in test failure when the main test thread goes into locking
it again. So give it a bit more delay after thread creation
so thread_12 has a chance to lock the private_mutex.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>