Prefer actual-expected to expected-actual in assertions

[radoering]

Relates-to: #4776

As far as I know, there is no general convention about the order (actual-expected vs. expected-actual) in pytest assertions. However, I think a consistent order is desirable in order to quickly grasp what is going wrong in a failing test.

In my opinion, there is at least one strong argument for actual-expected:
SIM300 of flake8-simplify is about avoiding yoda conditions. Thus, age == 42 is preferred to 42 == age. Avoiding yoda conditions often results in actual-expected order.

Further, most examples in pytest documentation are in actual-expected order, e.g. https://docs.pytest.org/en/6.2.x/index.html. (The same applies to unittest: https://docs.python.org/3/library/unittest.html) Probably, actual-expected is the more natural or at least more common order.

I am not aware if there is a linter that can help to keep actual-expected order because sometimes the context is required in order to decide which is actual and which is expected. At least, if most of the tests use a consistent order it will be more likely that this order is used in new tests.

[finswimmer]

LGTM 👍

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