[rubygems/rubygems] unset XDG_* environmental variables for testing of rubygems.

https://github.com/rubygems/rubygems/commit/539fd9a39a

lib/rubygems/test_case.rb

@@ -296,6 +296,9 @@ class Gem::TestCase < Minitest::Test
 
     ENV['GEM_VENDOR'] = nil
     ENV['GEMRC'] = nil
+    ENV['XDG_CACHE_HOME'] = nil
+    ENV['XDG_CONFIG_HOME'] = nil
+    ENV['XDG_DATA_HOME'] = nil
     ENV['SOURCE_DATE_EPOCH'] = nil
     ENV["TMPDIR"] = @tmp