Fix post policy for PHP 8

[clarkwinkelmann]

Fixes #3144

Changes proposed in this pull request:
Add unit tests and fix post policy for PHP 8

Reviewers should focus on:
I'm really not sure if that's the best way to built the tests, but it does cover the original issue.

Would this best be covered by a test of the edit post API endpoint directly instead of testing just the gate?

Should I namespace the test differently?

Should I use more test methods instead of grouping the time tests?

Screenshot

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[clarkwinkelmann]

I guess I could make the tests a tiny bit faster by re-using the models instead of calling findOrFail() each time. In particular the same user which is retrieved 4 times per test without any real need. I could also do the same for the posts but there's less gain there with 2 reuse per post. That's still a lot less DB requests. What do you think?

[askvortsov1]

LGTM! I think I would prefer variables referring to the model instances. There is a small performance improvement, but I think it'll also make the code a bit more readable.