[TEST] improve code quality of tests

[mhujer]

I've started improving the code quality of the tests. I'm doing it step-by-step in separate commits, so it can be easily reviewed.

I have a few other improvements in my backlog (using ::class instead of stringish classnames, adding type checks where appropriate, maybe some other things). @polyfractal do you prefer to have them all in one bigger PR or in more smaller PRs?

I plan to (eventually) do similar fixes for the src/ itself, but I wanted to start with the tests, because one does not have to worry about the BC here.

[polyfractal]

Great, thanks! My main working laptop died last week, so I'm still re-setting up my environment. I'll try to get everything setup and review this today/tomorrow. A quick skim looks good though!

do you prefer to have them all in one bigger PR or in more smaller PRs?

Let's go with smaller PRs, they are easier to review that way and we can get stuff in incrementally.

Thanks again for this! The unit tests have definitely been neglected, I tend to rely on the yaml REST tests for the most part :)

[polyfractal]

Also, just so you know, some of the yaml REST tests are failing for unrelated, upstream issues. So don't worry about them overly, we're waiting on fixes in core Elasticsearch.

[polyfractal]

Merging, thanks again for this!