test/unit: assert_raise_with_message

* lib/test/unit/assertions.rb (assert_raise_with_message): move from
  test/fileutils/test_fileutils.rb.  this is still experimental and
  the interface may be changed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41790 b2dd03c8-39d4-4d8f-98ff-823fe69b080e