gmake.mk: order test-all and test-knownbug

* defs/gmake.mk (test-all, test-ruby): depends on test-knownbug if
  check is given or with test-knownbug.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39964 b2dd03c8-39d4-4d8f-98ff-823fe69b080e