[Coco] video modes for diagnostic cart

[Alex Evans]

I have gotten to the point where I am currently working on the video test
for my diagnostic cart.  For video tests I have covered all (13) of the
coco1/2 video modes and for the CoCo 3 code to test the ones that are
implemented in the GIME. This kind of complete coverage for the GIME seems
unreasonable. I see two options: 1) implement a reasonable subset of the
GIME modes in the test (which ones?) or have a GIME subtest that allows the
user to play around with the parameters to see the results. Which do you
all think would be better?