Assist AB Testing mission with flipping users between variants

@arelra and @Jakeii did a preliminary look into this and think this is a good way to achieve this:

- Add a new cookie with a 24 hour expiry that flips between a value of “A” and “B”
- Hard code the editorial AB test name and variants in the VCL
- The test name and variants in dotcom-rendering will need to match the VCL
- If this cookie is present also lookup the test variant in the gu_ab_test_groups dictionary and apply it to the ab test header

Although this is a bit of additional VCL and involves hard coding the test name and variant, it will still be part of the framework by virtue of utilising the same dictionaries and X-Gu-Server-AB-Tests header

It would be good to pair with the ab testing devs to share ab testing and VCL knowledge