common.mk: RUN_OPTS to tests

* common.mk (btest, btest-ruby, test-knownbug): add $(RUN_OPTS) to
  ruby to be run, so that tests are runnable before making exts.
* common.mk (test-sample): ditto, and use $(MINIRUBY) as rubytest.rb
  does not need extension libraries.
* tool/rubytest.rb: pass $(RUN_OPTS) to testing ruby using --run-opt.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42989 b2dd03c8-39d4-4d8f-98ff-823fe69b080e