Added a test for monochrome encoding.

The test encodes 5 frames of a video using the --monochrome flag and
verifies that the decoded frames satisfy:

- each frame's monochrome flag is set to 1
- each frame's U and V planes are set to a constant, and this constant
  is the same for all decoded frames
- the initial frame's Y PSNR value is 'high enough'
- the Y PSNR values remain fairly constant across all of the frames

Change-Id: I4239ddfb745ed9746547737b4bc99963c71e51c0