Add automated test involving video capture hardware

This is a nacent attempt to write some automated tests to ensure that stb-tester continues to work with all our current supported hardware.  It's currently very basic but works.  The intention is that in the future we will have a test rig such that all commits are tested with real hardware to prevent future regressions.

The test is currently very basic, it just checks to see if something major goes wrong but in the future we can add greater control over the input to the hardware (e.g. using a Raspberry Pi) to measure other properties such as quality, latency, startup/shutdown time, ability to recover from video loss, ability to handle HDMI renegotiation, etc.