unit test oauth2 login

write unit tests for oauth2, use google as default test.

 - make sure `tests/config.php` details don't get commited (see [Contribugin.md](https://github.com/daithi-coombes/api-connection-manager/blob/master/Contributing.md) )
 - try test js window.opener report error is working/testable
 - make sure that caller window error reporting (as in wp-login page) is working/testable