Add a test to make sure SimplePushURLs is called correctly.

Test about the quick fix made here: https://github.com/mozilla-services/loop-server/commit/3a367ea39dc92bf7bd933a4fd610ce76afabc3c9#diff-d21dacc27858f10722004920c100877bR251
I believe this should be done in a separate test. The fist one here tests that it "should return the caller data", and we should add one that tests that the "simple push url" is being called.

This way, when one test fails, the other will not.