The screen flickers a bunch, so something involving my display is certainly happening.

As for runtktests...test suites that live in the package as opposed to the test directory require support files to gather the tests to be run.  This can be done by the test_xxxx.py file in the test directory (which is what regrtest runs), or it can be done in a function imported from the packages test directory.  For tkinter it looks like the test gathering is done in runtktests.  Most Python test_xxxx file can be run directly to run the relevant tests, and runtktests works the same way.  So yes it is a runner, but that appears to be a convenience just like for other test_xxxx files, and not anything special.