Support for specifying build flavour in test cases

[hndmrsh]

It would be cool if Plunge could test specific URLs only for particular build variants. By using manifest merging, I can ensure that some links are only handled by a certain build variant - for example, I only want to catch a "staging.example.com" link in debug builds, and not production builds. However, with the way the testing is currently done, there's no way of doing this.