docs: plugin tests & minor improvement

[Exirel]

Description

After discussing #2065 it appears that "how to test a plugin" isn't well documented, so here is a PR for that.

I think there are more that can be written, in particular about factories and how to use them.

I also added a convenient method to clear backend.message_sent, so you don't have to do it manually. It's useful when you have a bunch of automated messages, or when you don't want to write more than one test method for different yet very similar use cases (for example: trigger by a regular user vs admin vs owner).

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[dgw]

I love these additions! Did my usual thing, on mobile though (so not a great view of indentation or line length). Adjust suggestions as needed to wrap properly, etc.

[Exirel]

I love these additions! Did my usual thing

Thanks! Will fix that tomorrow.

[dgw]

Is that it? Are we done after just these two last nitpicks, or is Exi going to dump another 500 lines on me in a few days? 😁

[Exirel]

Is that it?

Yeah. I feel like I could write documentation for the Trigger factory, but I realized that it's almost an internal tool made to test some very specific part of Sopel's code. So I think I won't add (at least for now) a full-length explanation about it, and just leave the reader with the current docstrings, and having to look at our test suite.

That's good enough for me!

[dgw]