common.mk: use RUNRUBY for test-sample

* common.mk (yes-test-sample): use RUNRUBY instead of MINIRUBY to set
  runtime library path and run the built ruby.  [Bug #8971]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43129 b2dd03c8-39d4-4d8f-98ff-823fe69b080e