* bootstraptest/runner.rb (assert_normal_exit): check MJIT first
  to support btest with ruby ~2.5. btest (bootstraptest) should be
  enable to run with stable ruby interpreter because modified ruby
  may not able to run runner.rb and we need to know why (this is why
  we introduce btest).


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66842 b2dd03c8-39d4-4d8f-98ff-823fe69b080e