Make mcheck tests conditional on GLIBC_2.23 or earlier

Targets with base versions of 2.24 or later won't have
__malloc_initialize_hook because of which the tests will essentially
be the same as the regular malloc tests.  Avoid running them instead
and save time.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>