Add driver to testkit

[fbiville]

⚠️ testkit is not ready for public consumption yet, at the time of writing ⚠️

testkit is a test suite run by all official drivers, to make their behavior is as uniform as possible. There are undergoing works to make it easier to configure and use for an external driver, but this is not ready yet.

I am creating this issue so that this driver can be added once testkit is ready.

[transistive]

For this first iteration, the goal is to get familiar with Testkit and implement some basic tests for the PHP driver at first.