tests: kernel: re-path mslab test

tests/kernel/mem_slab --> tests/kernel/mem_slab/test_mslab
For the purpose to hold other mslab test apps.

Jira: ZEP-1209

Change-Id: I4a72b02a0a5095bb7cfbf73396b6d003ea63f92e
Signed-off-by: Sharron LIU <sharron.liu@intel.com>
6 files changed