* lib/test/unit/assertions.rb (assert_raise_with_message): set
  caller's last match if expected is a Regexp.

* test/ruby/test_arity.rb (err_mess): use assert_raise_with_message.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44172 b2dd03c8-39d4-4d8f-98ff-823fe69b080e