This commit fixes a number of missing cleanup steps in the unit tests
that kept state alive for longer than necessary:
- the loading tasks were not all being destroyed;
- the find controllers were not being reset;
- the state set in `beforeAll`/`beforeEach` was not all being nulled in
the correspoding `afterAll`/`afterEach` blocks.
Combined this resulted in a steady increase in memory usage of the test
process as the tests ran, climbing up to ~1.5 GB. After this patch the
memory usage remains stable at ~800 MB.