subsys/testsuite/ztest: change zassert_mem_equal to macro

This commit changes zassert_mem_equal as a macro instead
of a function implementation.
In the previous implementation when an assertion fails
the location inside ztest_assert.h file was displayed.
This modification displays the location where zassert_mem_equal
was used.

Signed-off-by: Radoslaw Koppel <radoslaw.koppel@nordicsemi.no>