Flesh out edge cases for each mutation test case

[dviator]

I'm creating some basic tests to check end to end flows of app usage, such as register -> addSource -> queryFeed.

It would be great to flesh out each intermediate test and helper function so that it checks as many cases as possible.

For example, instead of registering one user and adding a source for him, eventually the "Register" test could register many users with unusual info, with conflicting emails, and test the edge cases in addition to the happy path.