it would be very easy for a beginner to miss the last step of reverting to a specific plan at the end

Yes, that's true - it's fine to leave it out, of course.

mention t as a directory for tests

An alternative to explaining how to set up a t directory might be to just mention prove's default behavior when not given a filename, something along the lines of "You can give prove a list of test files to run, or if you don't give it any filenames, it'll look for files matching t/*.t and run those." - that might give enough of a hint.