fix(test): Add a test for signin unblock in Fx >= 57.
fix(test): Add a test for signin unblock in Fx >= 57.

The test specifies /settings, but the user should go to
/signin_confirmed. This is a bug that I haven't figured out
yet.

mozilla/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server