* Implement warning diagnostic flags for RGBFIX
* `-m/--mbc-type help` prints to stdout
* Support `-m list` as well as `-m help`
* Make invalid `rgbfix -l` characters a fatal error, like other invalid CLI arguments
* Refactor fix/main.cpp into multiple files
* Avoid redirecting error messages in RGBLINK tests
We check that RGBLINK prints nothing to stdout, so all that
would be captured on stdout is a putative failure message.
* Require each RGBLINK test to check error output
Otherwise, you can have a test that just... checks nothing!
* Document how to add tests
Fixes#1300's last remaining item.
Also add `checkdiff` rules to remind us to update that doc if
any of the test driver scripts are updated.
