Cleanup and refactor the tests

[nolar]

Problem

Over the year of active development, the tests are in a clumsy state with lots of inconsistencies. It is time to refactor them (with one or more iterations).

Proposal

The following is a collection of things to improve. Feel free to add ideas to comments.

• Run e2e tests in pristine-clean environments. Revise the e2e testing for pristine-clean environments #253
• Get rid of time-dependent tests. Use mocked time. Stabilise the time-based tests with asyncio sleeps/timeouts #212
• Get rid of mocks where possible. Use dependency injections.
• Add the tests for version upgrades.
• Extract the testing tools as project-local plugins, add type-checks to the tooling (not exposed for the users like kopf.testing!).
• Wrap various dicts/values in tests with proper classes, to remove IDE typing warnings.
• (?) Add typing to pytest fixtures in the tests (for IDEs).
• (?) Run mypy over the tests (non-strict, perhaps).
• Switch assertions of function calls to assert ...call_args_list == [call(*args, **kwargs)] syntax (now: assert k8s_mocked.sleep_or_wait.call_args_list[0][0][0] is None)