fix always succesful memory test

Since 51209b1f ("Use common mtest iteration counting"),
do_mem_mtest has always reported 0 errors and hence returned 0, even
if errors were detected. Fix the helpers mem_test_alt() and
mem_test_quick() to return the number of errors found.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>