tests: sched: preempt: Fix the skipped testing when enabling SMP

When SMP feature is enabled on board, this test suite will skip.
Enable the test suite by removing the filter.
Note that these testings only run SMP platform on single core
which are handled by setting MP_NUM_CPUS=1 in prj.conf.

Signed-off-by: Yinfang Wang <yinfang.wang@intel.com>