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

test/fileutils/fileasserts.rb

@@ -86,6 +86,15 @@ EOT
 File modes expected to be equal:
  <#{'%0*o' % [width, mode1]}>: "#{file1}"
  <#{'%0*o' % [width, mode2]}>: "#{file2}"
+EOT
+      end
+
+      def assert_ownership_group(expected, file)
+        actual = File.stat(file).gid
+        assert expected == actual, <<EOT
+File group ownership of "#{file}" unexpected:
+ Expected: <#{expected}>
+   Actual: <#{actual}>
 EOT
       end
     end