* 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.
