[5.6] PHPUnit 7

[carusogabriel]

Following #22998 with green tests

[taylorotwell]

@sisve can you describe in more detail your problems with the framework using PHPUnit 7 for its test and how exactly that affects package developers?

[sisve]

There wouldn't be any problems with this if we're still officially support phpunit 6 for package developers. I'm not sure that this would affect anything at all.

My questions in the previous PR was mostly why we should upgrade. What's in phpunit 7 that we need, and will it affect any package developers? Will we accept scalar typehints to other parts of the system after this PR?

Just say that we'll still support phpunit 6 for package developers so that it is known. It's not obvious after the previous discussions.

[taylorotwell]

How package developers test their packages seems entirely orthogonal to what the framework itself uses unless I'm still missing something.