Plexus-utils is used everywhere on Maven but is a legacy component. Most of its features is available using apache commons.
plexus-utils-commons-bridge is a compatibility layer to ensure backward compatibility, so that we can deprecate plexus-utils
Pick a plexus-utils class, and start creating tests... better still write the tests black box (that's what I am doing!)
Then when you have some tests written in the TCK module, create the implementation class with all the methods:
throw new UnsupportedOperationException("Not implemented yet!");
and then you can knock off implementations and see your test pass rate rise in the bridge module.
To stake your claim, commit the test case class first and then (unless you tell us otherwise) you are working on that class!