Add unit tests to PHP Meterpreter

[adfoster-r7]

Let's add unit tests to the PHP Meterpreter, similar to the recent addition of the Python Meterpreter testsuite - #651

Importantly: The test suite should work with all versions of PHP to 5.x. For instance, if using PHPUnit it should be possible to use the runner on older versions of PHP. It was for this reason we used Python's inbuilt unittest test framework.

Usage of PHPUnit may or may not be viable - supported versions: https://phpunit.de/supported-versions.html