[proposal] Use PHPUnit static assertions in AssertionsTrait

Since the AssertionsTrait now depends on inheritance from PHPUnit, it is not possible to use it outside of PHPUnit test cases. With this simple change, the trait can easily be used together with other testing frameworks, such as Behat.

Ps. I'm not sure if this is the right way to do proposals like this, and I understand that there have been some changes. If not, please point me in the right direction and I'll make the changes necessary.