[6.x] Restore phpunit 7 support

[GrahamCampbell]

Closes #31101.

[spawnia]

Thank you. I really dislike the direction that PHPUnit has been going in lately with locking down classes/methods to be impossible to extend.

[GrahamCampbell]

PHPUnit 7 support will be removed in Laravel 7, however.

1. Laravel 5.8 supports PHPUnit 7 and 8.
2. Laravel 6 supports PHPUnit 7, 8 and 9 (we are treating it as a bug that support for PHPUnit 7 is currently not present in Laravel 6, and this PR will restore it)
3. Laravel 7 will support only PHPUnit 8 and 9, when it is released in a few weeks.

The reason for saying Laravel 6 will support PHPUnit 9, even though it's not out yet, is because Laravel 6 is an LTS release, and so needs to work on PHP 8.0 and 8.1 when they come out. There's a good chance PHPUnit 7 or 8 won't work properly on PHP 8, but PHPUnit 9 will likely get any fixes backported to it from PHPUnit 10.

[driesvints]

@GrahamCampbell any chance we can get the tests fixed so we can get this merged? I think there's just a copy/paste gone wrong. The Assert class seems to be placed twice inside the same if statement.

[lukasjuhas]

Thank you for this!

[GrahamCampbell]

Added that missing line. Looks like the tests were broken on PHPUnit 7, since the exception assertion stuff isn't present.