The library manual has chapters for

unittest: the generic testing framework (package) that any app can use

test: a package with the Python test suite; test_x tests module x
  It has other modules and subpackages, probably not all documented.
  The test modules use unittest, doctest, and other code.

test.support: some of the other code

AS it says in the test chapter, the test suite imports each test_x and runs its test_main function. The tkinter package includes a test subpackage. test_tk checks that _tkinter and tk are available and if so, runs the tests in tkinter.test. (But I am not sure it really does what it should ;-).

My thought is that we might want to do something similar with idle. However, there needs to be a test_idle module in any case, so we can start with a few tests there and migrate them to a separate idlelib.test subpackage later if we want.