[5.8] assertArraySubset deprecated in PHPUnit 8

[GrahamCampbell]

Either we need to convince PHPUnit to revert this, or find a replacement, otherwise test code using Illuminate\Foundation\Testing\TestResponse::assertJson will start to throw deprecation warnings.

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Relates to sebastianbergmann/phpunit#3494. // cc @driesvints

[driesvints]

No worries. I already took care of this in #27441. I've committed 695a299 which overwrites the method to prevent the warnings. I couldn't take over the full ArraySubset class because the method signature on the Constraint base class was changed and thus it will only work on either PHPUnit 7 or below or PHPUnit 8 or above. We'll take over the full class as soon as we're dropping support for PHPUnit 7 and add support for PHPUnit 9 in February next year.

[driesvints]

Thanks for mentioning btw! :)