[Piglit] Plan for enabling more tests in concurrent mode

[Kenneth Graunke]

I agree with Paul: I'd rather see the keyboard-driven tests be converted
to take mandatory command line arguments and only test/display exactly
one thing.

I also think they should list the possible options if you don't give it
the option (i.e. a help message).  With fbo-generatemipmap-formats, for
example, I can never quite remember the name of the extension that does
S3TC + SRGB, but given a list I could find it quickly.

I'm not a huge fan of a grid.  That's a decent option for tests with
only a few combinations, but it gets quickly out of hand.  I honestly
doubt that fbo-generatemipmap-formats would fit even on a 1080p screen. :)