Add functional tests to our JSON API

[pascalchevrel]

We have unit tests but those can't expose errors caused by thiungs such as files permissions, url rewrites and other factors in the environment. Unit tests also give us results on how our classes work but not on if the app believes correctly, it doesn't test the actual end result.

For the JSON API, this can be a problem because we don't want to regress how it works while adding new features. I have code that allows testing that the Json API is behaving as expected.