Strange.  test_idle runs fine on my Mohave, so I cannot investigate what you see.  Tests of classes that create windows and call wait_window pass _utest_=True and wait_window calls are guarded by "if not _utest:".  I believe this is always in __init__.  You could grep idlelib for _utest, find a window that stays open, and add prints to see what tkinter call is causing the hang, and if it is in the not-_utest suite.

When I run the entire suite rather than just test_idle, I instead get multiple "Python quit unexpectedly" boxes and failures in 5 other tests.