doc: Add ability to generate test suite documentation

Add a new 'doc-test' Makefile target which builds documentation for the HID tests. Having access to this documentation in a more easily-browsable form may be useful for those writing new test harnesses from scratch. We provide Sphinx with the --private flag since these particular docs are not intended for third-parties anyway.

Signed-off-by: Jason Gerecke jason.gerecke@wacom.com