test_planner is for creating expectations about the planner from the perspective of downstream components (for example: the executor).
It defines an extension that can be installed into a running instance. The extension defines a function that runs a suite of expectations about the planner.
make installcheck
During development, you can get feedback on success and failure
make test
Also, the extension can be installed and run as a query:
make clean install
create extension test_planner; select test_planner();