Add tests for the update manager

forgot the tag

Note, unit tests specifically for the Updater classes can be handled at #602544: Write Unit tests for the Updater classes . This should be for stuff like the update manager UI. Perhaps we should split off tests for authorize.* into a separate issue, not sure yet...

Note, there are some initial tests for authorize.php itself here:
#609772: Impossible to extend the FileTransfer class system in contrib
I've also added some Update manager tests here:
#700558: Add test to ensure filename extensions are properly merged in the Update manager UI
#931284: Update status admin UI shouldn't rely on hook_help()

However, there's still a ton to do in here. First step is here:
#933392: Add tests to verify that you can plug in your own Updater class
That'll give us the basis for writing tests, since we can use a fake backend to do "upgrades" in a controlled environment, instead of trying to talk to FTP, etc.

Plus, we should test the Update manager UI itself..