Ignore `XDG_CONFIG_*` environment variables in tests

This avoids polluting the user's home environment.
https://github.com/github/hub/pull/1705#issuecomment-392759921

features/support/env.rb

@@ -36,6 +36,9 @@ Before do
   set_env 'GIT_PROXY_COMMAND', 'echo'
   # avoids reading from current user's "~/.gitconfig"
   set_env 'HOME', File.expand_path(File.join(current_dir, 'home'))
+  # https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html#variables
+  set_env 'XDG_CONFIG_HOME', nil
+  set_env 'XDG_CONFIG_DIRS', nil
   # used in fakebin/git
   set_env 'HUB_SYSTEM_GIT', system_git
   # ensure that api.github.com is actually never hit in tests