Bug 1079950 — Add a test to make sure GET /calls returns the callerId if present.

https://bugzilla.mozilla.org/show_bug.cgi?id=1079950
yeah, I concur, the way we're testing is not consistent at all: we should make a call here between functional tests and unit tests, and stick with it whenever possible, as this will help a lot in these scenarios.

Let's keep it like that for now then, and we'll change that later on, since it's already inconsistent.