test_gem.rb: exclude default gems

	* test/rubygems/test_gem.rb: check difference to exclude default
	  gems from the expected results.  [Feature #13186]
	Merge rubygems-2.6.11

	  This version fixed regression of rubygems-2.6.10.
	  https://github.com/rubygems/rubygems/pull/1856

	  See details of changelogs for 2.6.11 release:
	  https://github.com/rubygems/rubygems/blob/adfcf40502716080bd9cdfdd2e43bd4296872784/History.txt#L3

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@58001 b2dd03c8-39d4-4d8f-98ff-823fe69b080e