Tests: implement work-around for remaining PHPUnit cross-version issue
Tests: implement work-around for remaining PHPUnit cross-version issue

PHPUnit 9 removed the `assertInternalType()` method in favour of more specific methods, which were introduced in PHPUnit 8.

As this test suite only uses this assertion in one place, putting a polyfill in place is over the top. A simple if/else does the trick.

Yoast/whip