subsys/testsuite/ztest: Fix zassert_mem_equal user message printing

This commit fixes the zassert_mem_equal macro to properly print
the formatting string is given as a parameter by the user.
There is an error that is an effect of directly copying the
previous implementation that was using the inline function.

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