test: atomic operation : add code description

The existing testcase's doxygen describes are the general
implementation idea of a function.On this basis, adding
more descriptive statements to describe which conditions need
to be preset when running the testcase, which test techniques
are applied, and describe the testcase Design steps in detail.
Make it more readable.

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