tests: add ending logic for the preempt test

Old tests/kernel/sched/preempt cannot be run repeatedly.
If you register the test_preempt() method twice, the
second run will fail. It is because the participating
threads don't exit properly when the main thread exits.

The new ztest fx implies that a unit test should be able
to run repeatedly. This commit enables that for the preempt
test by adding explicit epilogue.

Signed-off-by: Ming Shao <ming.shao@intel.com>