skip SEGV (BUG) tests if ENV['RUBY_ON_BUG'] is given.

This environment variable can show additional message on BUG.

test/ruby/test_rubyoptions.rb

@@ -718,6 +718,8 @@ class TestRubyOptions < Test::Unit::TestCase
   end
 
   def assert_segv(args, message=nil)
+    skip if ENV['RUBY_ON_BUG']
+
     test_stdin = ""
     opt = SEGVTest::ExecOptions.dup
     list = SEGVTest::ExpectedStderrList