Update #localize to use LIST special-use format

The XLIST extension has been depreciated by Google, with the replacement being
the LIST special-use extension. Syntax is almost exactly the same; the only
two changes are:

  * Flagged now denotes Starred messages
  * Spam is now called Junk

Otherwise, operation should be exactly the same.
Move mock_client method to spec_helper

This is used by tests outside of the client_spec so I moved it to the
spec_helper file.