Add test coverage for bad test object variables & edge use cases

[kingo55]

Our test coverage is quite decent right now, but I think we could add a couple of unit tests to the suite for edge case scenarios like:

• Tests do not activate more than once per page
• Preview mode test activates properly
• When the chosen recipe no longer exists in test object, exclude the user
• Recipe sample rates <> 1, or not set on all recipes