Add a Persona/123done test to check B2G retains Persona cookies

This contains a Persona smoke test to check that Firefox OS is retaining cookies

I've also updated the Mock object and after this has been merged I will modify the Marketplace to use the correct type of Persona user each time (for example Login can use a verified user but payment should use an unverified user).

---- branches:
`v1-train` -> Tested and can be merged
`v1.0.1` -> Tested and can be merged
`desktop` -> Untested