Run test_tkinter in CI

[geofft]

From src/verify_distribution.py:

    @unittest.skipIf("TCL_LIBRARY" not in os.environ, "TCL_LIBRARY not set")
    @unittest.skipIf("DISPLAY" not in os.environ, "DISPLAY not set")
    def test_tkinter(self):
        import tkinter as tk
        ...

We ought to be able to run this in Xvfb and ensure it runs, though we'd have to automate clicking the button, either by injecting a button click or by modifying the test to quit on its own.

(It looks like Tk is pretty heavily tied to X11 at the moment, but if we feel like being complete, we could also test under Xwayland + some headless Wayland compositor like WLR_BACKENDS=headless sway or weston --backend=headless.)