Add UI Tests mozId tests

Add test for UI Tests > navigator mozId > standard persona sign in
In persona/app.py
- make continue button css more explicit
- refactored continue button tap, as I was having problems with timing, now it follows the format.

Testing Notes:
- Passing on Inari with 1.2 (today's build)
- test_persona_cookie.py also passes.

@zacc : can you please review?