More asserts

Added a couple more PhpUnit assertions, the goal is that all PhpUnit ones have a Verify counterpart. 
Adding my ¢2 (nearly a month later)
While I agree that I've never used the file assertions in any project I've work on, I don't think it's really for me to what is or isn't useful to other people. The fact that these assertions are built into PHPUnit indicates to me that somebody somewhere makes use of them, so it would be reasonable for us to add support for those assertions in this wrapper.