Initial fate tests.

Keyframe-only for now (i.e. -frames:v 1), because keyframes are
not yet complete so they would just produce noise. References
were produced using vpxdec $$ -o test.y4m; ffmpeg -i test.y4m
-frames:v 1 -f framemd5 tests/ref/fate/vp9-$$.

Currently, the following tests fail for a variety of FIXMEs:
fate-vp9-00-quantizer-00.webm
fate-vp9-02-size-NxM.webm for several N/M combinations
fate-vp9-03-size-NxM.webm for several N/M combinations
fate-vp9-emu-edge-* (several additional ones beyond non-emuedge)
Add TODO list so we can track outstanding items easier.