fix(test): Fix "Firefox Desktop Sync v1 signin - unverified" functional test. (#5634) r=@philbooth
fix(test): Fix "Firefox Desktop Sync v1 signin - unverified" functional test. (#5634) r=@philbooth

Ensure localStorage state is really cleared before starting test.

fixes #5633
mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server