ExPlat: remove "unknown" variation, handle null and not-returned cases

[leandroalonso]

This PR implements some changes in the AB testing feature based on @withinboredom comments in a previous PR:

• Remove the unknown variation. The default fallback is control
• If the experiment has a null value in the JSON, treat it as control
• If the experiment is not returned in the JSON, treat it as control
• Removes the other() variation. All variations are effectively a treatment variation, the API was slightly changed:

            switch abTesting.experiment("experiment") {
            case .treatment(let variation) where variation == "variation-name":
                // do something
            case .treatment(_):
                // default variation
            case .control:
                // control
            }

How to test

Code review and green CI should be enough. :)

[withinboredom]

LGTM, but might want to also remove other as well, and just have treatment accept a name for the treatment.

[withinboredom]

It might be a good idea to do what @renanferrari did on wordpress-mobile/WordPress-FluxC-Android#1845 -- and make Treatment take a string since other() is just a different Treatment (for when we support multi-variate experiments).

[leandroalonso]

@jkmassel I have a totally unrelated Mac OS X test failing here (testThatOnlyOneFileIsUploadedSimultaneously).

Running it again doesn't seem to solve (and the test works just fine in my machine™️), any idea how to proceed?

Edit: of course now it worked.