[TASK] Allow assertInstanceOf calls in tests

[oliverklee]

We find those checks useful.

[coveralls]

coverage: 38.583%. remained the same
when pulling 6370fe4 on task/assertinstanceof
into fe2eaa0 on main.

[JakeQZ]

Looks like this just needs a baseline update to remove the instanceof error which is now squished by the config change...

[oliverklee]

Looks like this just needs a baseline update to remove the instanceof error which is now squished by the config change...

I tried that: It turns out that we currently do not have any of those warnings (I tend to put this on the fact that PHPStan is still at level 1 for this project), and blocklisting warnings we don't have (yet) is something that PHPStan does not support.

So I'll keep this PR sitting here for the time being and un-draft it once we actually have any of those warnings to ignore.

[JakeQZ]

Looks like this just needs a baseline update to remove the instanceof error which is now squished by the config change...

we currently do not have any of those warnings

Ah, I see. I misunderstood the error message (did not read it properly), and thought it was referring to an existing error that now needed to be removed from the baseline.