test_fileutils.rb: tests for chown

* test/fileutils/fileasserts.rb (assert_ownership_group): new
  assertion for group ownership.
* test/fileutils/test_fileutils.rb (test_chown{,_verbose,_noop}):
  based on the patch by vajrasky (Vajrasky Kok) at
  [ruby-core:59281].  [Feature #9286]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44364 b2dd03c8-39d4-4d8f-98ff-823fe69b080e