Now that we're pointing at `src/` for tests, we can stop trying to load source maps from random places. With this dependency used, source maps are off by a few lines.
The earlier commit, d3ce0cb82f, has most of the juicy details on this. In addition to d3ce's changes, we also:
* Use `TestClient` in many integration tests due to subtle behaviour changes in imports when switching to ES6. Namely the behaviour where setting the request function is less reliable in the way we did it, but `TestClient` is very reliable.
* We now use the Olm loader more often to avoid having to maintain so much duplicate code. This makes the imports slightly easier to read.
