Migrate to a new testing framework and improve tests

[outoftime]

Testing has become a real pain point. There are two related issues here:

• tape and karma-tape just aren’t very usable. The most acute pain comes from the lack of file/line information in test failures; a close second is the basically unreadable error output.
• I’ve made some mistakes in the way I have structured the tests. Specifically:
  • The reducer tests use the reducerTest helper function, which takes a before-state, an action, and an expected after-state. This however does not match the way one naturally thinks about testing reducers, namely: start with this state; apply this action; then make specific assertions about the resulting state (rather than just saying “this is what the resulting state should be”).
  • Reusable test helpers, particularly for creating test data, are unintuitive and not well-structured. We should have factory modules for each category of object we’d like to create: e.g., one for objects that the Firebase SDK would give us; one for objects that the GitHub API would give us; etc. We should use something like rosie to make factories simple to build and easy to extend.
  • Test code should not do direct manipulation on Redux state data structures. Redux state used in tests should always be generated by running a series of actions through the reducers. We should have a helper module to make this boilerplate-free.

[outoftime]

This is basically a duplicate of #1443, moving relevant writeup over there.