maint.mk: add syntax check for use of compare from init.sh

maint.mk: add syntax check for use of compare from init.sh

Comparing expected against actual gives more consistent diff listings
when reporting test failures.  Enforce this idiom on test files
that use init.sh, and allow projects to recognize an alternate
pattern for recognizing tests scripts that use a compare function.

* top/maint.mk (sc_prohibit_reversed_compare_failure): New rule,
moved here from coreutils.

Signed-off-by: Eric Blake <eblake@redhat.com>