modetest: add manpage

Add manpage for the modetest tool.

The following discussion from !284 (merged) should be addressed:

• @lumag started a discussion: (+1 comment)

  @narmstrong @MarijnS95 an alternative option (though much more complicated) would be to add a manpage for modetest and have the BNF data in the manpage.

added modetest label

added tests label

I've recently(for reasons) ended up reading through bits and pieces of the Linux Kernel DRM stack and have come to realize that I can never remember how to invoke modetest. I'm happy to work on a manpage for modetest should that make invoking it easier. Is this still a priority?

It's not a priority, but definitely a very welcomed addition to libdrm.

Awesome! Also, would adding tab completion be OK? Often times, to see what option are available, I run modetest -M some_drive -D /dev/dri/cardX -c which spits out a massive list I have to sort through.

Tab completing could ease that experience a bit. Could be especially handy for quickly iterating when testing drivers or debugging silicon IP.