Use flarum/testing for test infrastructure

[askvortsov1]

**Ref #2543 **

Changes proposed in this pull request:
Delete old testing infrastructure in favor of that in flarum/testing

Confirmed

• [ ] Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

[clarkwinkelmann]

Globally this seems all right.

I'm just curious where the temporary directory went? I see there are some system variables for it in flarum/testing. Is there a gitignore for the default location if those system variables aren't set? Is that tmp folder created inside of vendor/flarum/testing by default? (__DIR__ . '/tmp https://github.com/flarum/testing/blob/main/src/integration/UsesTmpDir.php) That doesn't seem right

[askvortsov1]

Is that tmp folder created inside of vendor/flarum/testing

It is. There's some discussion on this in #2544.

[clarkwinkelmann]

Ok I didn't see that discussion. Looks fine then, let's go ahead with this.