Test documentation

[kaskranenburgQ]

This PR adds the general vision for testing, along with a more detailed plan for improvements for the mechanical turk tests.

[mabijkerk]

Thanks for this setup @kaskranenburgQ. I have made some changes but mainly want to discuss the following points:

• Should this documentation replace the Mechanical Turk README or be complementary to it?
• I would not include the proposed improvements for the Mechanical Turk in our documentation, those work better as issues on the repo itself. Instead, I think it would be useful to include a list of type of tests that we can run with the Mechanical Turk: checking charts, checking matching hourly and yearly values, checking the expected result of a slider etc.
• If we're actually going to add all the testing tools to the documentation, then I doubt whether we should include this all under "Writing tests", or whether a dedicated sidebar item would be clearer.