sbin/md5: disable tests due to build failures

Testing with buildworld/installworld had succeeded, but it appears
that a path is not correctly set for the distribute target in the
tests directory.

Commenting out HAS_TESTS should make the build succeed again and give
time to resolve the "make distribute" issue.

The tests have been locally run to verify that the recent changes to
add a coreutils compatible -c option does not lead to any regressions.