Use 'WITHOUT_TESTS=1' instead of 'MK_TESTS=no' in the
'distribute' target of etc/Makefile, because we do not
allow command-line use of 'make MK_TESTS=no' in stable/10.

This fixes a regression introduced in r270187 that causes
the release build to fail, and a direct commit to stable/10.

Sponsored by: The FreeBSD Foundation