fix(test): No longer redirect on the web channel tests.

@vladikoff - r?

The web channel tests redirect to the RP when the oauth_complete message is received, making it impossible to test that FxA is showing the correct screen when the flow completes.

* Use a similar strategy to the FxDesktop/Sync tests that listen for messages and add elements to the DOM. Test for the expected element's existence on the DOM.

Note - this update does not add the tests for the correct screen, that will come in a follow on PR.

fixes #1943