Convert all tests to pytest style

[decko]

Any attempt to create a new plugin should already use the pytest style for tests.
This could avoid any after rework converting tests.

[lubosmj]

It would be worth discussing whether those tests need to be maintained at all. As a plugin writer, I had to rewrite them from scratch. The tests are here as a reminder for plugin writers to ensure they do not forget to write tests. They are not executable by default. Have we considered removing them? We would only leave an empty dir tree there (with one empty file -> test_nothing.py).

[ggainey]

I think having at least one example of "this is how to write tests" is useful for a new plugin author, fwiw.