test_fileutils.rb: tests for chown

* test/fileutils/fileasserts.rb (assert_ownership_user): new
  assertion for user ownership.
* test/fileutils/test_fileutils.rb (test_chown_error),
  (test_chown_without_permission, test_chown_with_root):
  based on the patch by vajrasky (Vajrasky Kok) at
  [ruby-core:59298].  [Feature #9292]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44387 b2dd03c8-39d4-4d8f-98ff-823fe69b080e