test: atomic: multiple equal priority threads access atomic variable

Add an testcase. Creat two preempt threads with equal priority to
atomiclly access the same atomic value. Because these preempt
threads are of equal priority, so enable time slice to make
them scheduled. The thread will execute for some time.
In this time, the two sub threads will be scheduled separately
according to the time slice.

Signed-off-by: Ying ming <mingx.ying@intel.com>