Sniffing files in tests directories

[jaymcp]

Discussed in #166

^{Originally posted by g-elwell June 24, 2024}
We're currently excluding files in tests directories from being sniffed. There's likely a good reason for this, and we may want to enforce a different ruleset for these files than within our application code.

However, without any sniffing whatsoever, it's easy to hit formatting issues which would otherwise be avoided.

Further to this there may be test-specific sniffs we could implement. I haven't looked into anything at this stage, but as there are conventions which need to be followed in relation to how test files and methods are named, PHPCS may be able to help us out here.