Democratizing code fixes - part one

[psfinaki]

I am making code fixes great again, so that they are easy to write and to test.

This PR is the first step with the scope of 3 random code fixes. The changes include the following:

1. Adding some basic testing framework
2. Adding some tests
3. Adjusting the code in the code fixes
4. Rewriting asyncMaybe to cancellableTask
5. Fixing recently introduced Convert To Anonymous Record code fix is broken #15372 discovered along the way, to prove this all makes sense :)

Apart from fixing the bug, the code is left untouched as much as possible within the changed monads.

We'll see where things go - so this doesn't aim for perfection, other code fixes might required adjustments in the testing framework.