Rather than maintain our own test runner we should instead lean on the community and use Mocha which is very popular and also our test runner of choice in DevTools too.
Note that this commit doesn't remove the TestRunner source as it's still used for other unit tests, but they will be updated in a future PR and then we can remove the TestRunner.
The main bulk of this PR is updating the tests as the old TestRunner passed in contextual data via the `it` function callback whereas Mocha does not, so we introduce some helpers for the tests to make it easier.
They fail because cookies in Firefox return a `sameSite` key which the tests don't expect.
This is a solution that at least gets the Travis Firefox build (hopefully!) green again. Longer term it'd be great to allow the assertion to change based on the browser, rather than skip these tests entirely.
