docs(testing): update code samples in ops.testing from unittest to pytest style

[IronCore864]

Since we are encouraging charm tests to be written with pytest, we should use that in test code examples in ops.testing.

Only code samples from ops/testing.py are updated, other files/test cases are not updated.

Closes #1154.

[tonyandrewmeyer]

Looks good, thanks! A couple of questions about seemingly no-op indentation changes, and a suggestion about simplifying one of the examples.

[tonyandrewmeyer]

Yeah, I think this approach works nicely, thanks!

One small suggestion, and one place where I think the indentation isn't right.

[tonyandrewmeyer]

Thanks; looks good!

[tonyandrewmeyer]

While working on another ticket, I noticed that there's also this unittest style doc in testing.py that should be updated as well.

[benhoyt]

I noticed that there's also this unittest style doc in testing.py that should be updated as well.

Yeah, it would be nice if we could just tweak the text there to be more general. Something like: "Always call harness.cleanup() after a test completes (normally this is done within a pytest fixture)."

[benhoyt]

Let's just get that tweak to the cleanup docstring in and then this is good to go!