FileUtils::RUBY instead of fixed "ruby" so that the ruby command
works fine in Ruby's "make test-all".
(test_sh_with_a_single_string_argument): ditto.
(test_sh_with_multiple_arguments): ditto.
(test_sh_failure): ditto
(test_sh_special_handling): ditto.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@25385 b2dd03c8-39d4-4d8f-98ff-823fe69b080e