Add support for PHPUnit 7 or 8

[pascalheidmann]

Also maybe drop support for older PHPUnit versions (see #553)

[GrahamCampbell]

Can't drop older versions, without dropping older PHP. "support for PHPUnit 7 or 8" doesn't matter for users of this package, since it's a dev requirement.

[denis-sokolov]

Thank you for the suggestion, @pascalheidmann. As Graham has mentioned, our tests are internal and the users don’t see the difference, thus the motivation to change the tooling is very low. ¯\(ツ)/¯

[SvenRtbg]

Related: #504