Fix Testing section in README

[bwaidelich]

The Testing section referred to a non-existing composer script test

[bwaidelich]

Let me know if you want me to add the missing composer script instead and I'll push a follow-up

[Crell]

We've got a phpstan script, so may as well make one for tests. Although I rarely use them, as for reasons I've never figured out running a composer script when xdebug is enabled just hangs, at least on my Docker images. I've yet to figure out why... 😢

In any case, yes, let's add the script instead to match the readme.

[bwaidelich]

We've got a phpstan script, so may as well make one for tests

Done.
I did not adjust Makefile and quality-assurance.yaml to use the script since they weren't using the phpstan script either

running a composer script when xdebug is enabled just hangs

uh? I've never experienced that

[Crell]

uh? I've never experienced that

Yeah, I can't explain it either.

Thanks!