Refactor tests

[sjakobi]

…by moving the test definitions directly into the trees.

Context: #284.

This is also preparation for the addition for more invariant-checking tests (#449).

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TODO:

• HashSet tests
• More test suites?! Nah

[sjakobi]

I'll spare the other test-suites for now.

In the case of Properties.List I suspect that we won't need to do any changes and can hopefully get rid of them when #155 is implemented.

The regression test cases are mostly a bit larger, so I don't think they'd benefit from being inlined into the test tree.

I'm not very familiar with the strictness tests yet. I'll look at them at a different time.