test: unify assertEqual calls in all the tests

[pbodnar]

It's easy and unified now (with no exceptions, right?! :)):

1. As per unittest's documentation examples, actual always goes before expected.
2. If used, actual is stored in a variable output for us.
3. If used, variable expected is defined after the output variable.

For curious ones, Python (as well as the rest of the world) is not consistent here: https://stackoverflow.com/questions/40945126/parameter-order-for-unittest-testcase-assertequal.

[coveralls]

coverage: 91.086%. remained the same when pulling 1a718d5b8d0c3abcb7aa44dde8a2a38c28fcf72c on unify-tests-asserts-on-equal into faed4f3aeb18cd930d8b0d3b12417cbb7882cc91 on master.

[pbodnar]

@anderskaplan, this might be of an interest for you. :)

[anderskaplan]

Looks good to me! 👍