Add test suite for functional core of dashboard
The dashboard was created without any test suite, since we'd iterate fast on it and probably change things often.
But, with !39 (merged), we already reached a tipping point where tests would have helped - at least, for the functional core of data transforms and so on. The UI itself is relatively simple (and likely to change a lot still), so it doesn't quite make sense yet to add tests for that.
Thanks @thutterer for bringing this up in !39 (comment 1052130376)!