fix: get tests passing on plucky

[Perfect5th]

Twisted changed a little bit - it used to run cleanFailure on Failure instances in the defer code but no longer does in the version available in plucky (python3-twisted 24.10).

This change does cleanFailure in the FakeReactor we use for some tests. This allows tests that rely on catching and flushing Failures to succeed.

[Perfect5th]

One way to test, created a plucky container using dev_lxc (https://pypi.org/project/dev-lxc/) and run the tests, with and without this change:

dev_lxc create --config .dev-lxc/config.yaml plucky
dev_lxc shell plucky
make check

[wck0]

Tests passed on plucky, noble, jammy, and focal.

One in-line comment asking to review some grammar.

Also, codecov seems confused. Not sure how to address that...

[Perfect5th]

I'm not too worried about codecov. If anything, we should disable it in favour of TiCS.