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

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.

fixes #1943

jrgm/fxa-content-server,npestana/fxa-content-server,ofer43211/fxa-content-server,riadhchtara/fxa-password-manager,TDA/fxa-content-server,ReachingOut/fxa-content-server,atiqueahmedziad/fxa-content-server,vladikoff/fxa-content-server,swatilk/fxa-content-server,TDA/fxa-two-factor-auth,jrgm/fxa-content-server,ofer43211/fxa-content-server,riadhchtara/fxa-content-server,mozilla/fxa-content-server,chilts/fxa-content-server,ReachingOut/fxa-content-server,TDA/fxa-two-factor-auth,TDA/fxa-two-factor-auth,dannycoates/fxa-content-server,jpetto/fxa-content-server,riadhchtara/fxa-content-server,riadhchtara/fxa-content-server,dannycoates/fxa-content-server,dannycoates/fxa-content-server,mozilla/fxa-content-server,riadhchtara/fxa-password-manager,TDA/fxa-content-server,atiqueahmedziad/fxa-content-server,ReachingOut/fxa-content-server,swatilk/fxa-content-server,ofer43211/fxa-content-server,npestana/fxa-content-server,vladikoff/fxa-content-server,atiqueahmedziad/fxa-content-server,npestana/fxa-content-server,mozilla/fxa-content-server,jpetto/fxa-content-server,swatilk/fxa-content-server,jpetto/fxa-content-server,chilts/fxa-content-server,chilts/fxa-content-server,TDA/fxa-content-server,riadhchtara/fxa-password-manager,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,vladikoff/fxa-content-server