Make unit tests that use ThreadPoster test doubles faster

[techyourchance]

The current unit tests performance is acceptable for small and medium sized projects, but can be problematic for big projects that might have 1000+ multi-threaded test cases.

Some initial ideas to research:

1. Switch UiThreadPosterTestDouble to serial execution on a single thread.
2. Allow for sharing ThreadPoolExecutors among different test cases (e.g. by making them static).