HIL testing

Hi all

I've raised an issue for my initial concept for the HIL testing of the UART driver: https://github.com/RIOT-OS/RIOT/issues/8184.

I'm going to start on it immediately in order to go ahead with issue https://github.com/RIOT-OS/RIOT/issues/7941. All comments and input would be gratefully received while I'm doing so! Once the tests have been developed and used and any feedback discussed/integrated I'll open a separate PR for the UART HIL tests.

Perhaps we could aim for the discussion on this and the I2C HIL setup to feed into a standard implementation, to allow it to be more easily implemented in Murdock, if that would be useful?