tests: kernel/smp: mark torture test as skipped if factor is 0

If CONFIG_SMP_TEST_RUN_FACTOR is zero, the switch torture test
is effectively not doing anything as the k_sleep() below is not
going to sleep at all, and all created threads are being
terminated (almost) immediately after creation. So if run
factor is zero, mark the test as skipped.

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