plugin: example raises AssertionError when regex does not match

[Exirel]

Description

The decorator example generates a test function when used with pytest and an expected output. When a regex is used for said test, a fake assert expected == output was used to mark the test as failed. It is not adapted because a) if the assert pass, it means the test should not use a regex in the first place and b) it creates a confusing test output. So now, it raises an AssertionError with a proper error message, which is well understood by any test runner - after all, assert is a way to raise the exact same error.

Note: I use plugin: ... in the commit message because it's related to the sopel.plugin.example decorator, so it makes more sense to me.

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]

Honestly I'm surprised no one thought to make a change like this sooner. Test output is definitely confusing for regex examples with the plain old assert…