Add back some deprecated and general purpose API tests. These are the
ones that were written already pretty much conforming to the GTest API
and style, and thus require minimal porting.
For now, it just generates a simple horizontal slide (by writing
to /dev/uinput) and checks that the stage gets the events at the
expected coordinates.
The test won't run if it doesn't have read/write permissions to
/dev/uinput.
It also adds OS_LINUX to config.h.
