tests: kernel: mem_heap: added api and concept tests

<kernel.h> APIs covered:
    k_malloc
    k_free
Concept TESTPOINTs extracted from
https://www.zephyrproject.org/doc/kernel/memory/heap.html#concepts



ZEP-1242

Change-Id: I39edc809119984585d78d6abbe33f5be707c5818
Signed-off-by: Sharron LIU <sharron.liu@intel.com>