Additional notes:

I am puzzled that test_idle output starts with test_io, as that is alphabetically in the middle of the files in idle_test/.

There are 2 independent 'test_io' modules: test/test_io, which tests the io module, and idlelib/test_idle/test_io, which tests idlelib.PyShell.PseudoFile and subclasses (which are all derived from io.TextIOBase).  Only the latter is an issue here.  The former is an irrelevant distraction and should not have been included with test_idle in msg321737.

Since log.all ends with "test_writelines (idlelib.idle_test.test_io.PseudeOutputFilesTest) ... /usr/bin/xvfb-run: line 181: ...", I initially thought that test_writelines, in particular, stimulated the failure.  But in the msg321737 output, test_writelines ran 'OK'.

The fact that xvfv ... test_idle runs by itself but not when alphabetically preceding tests are run says that some preceding test alters something.   The failure of test_audioop seems an unlikely cause.  I suspect test__all__, by importing _tkinter either directly or via Tkinter or some IDLE module.  This could be tested with xvfb ... test___all__ test_idle.

Since the tests are mostly run alphabetically and the tkinter tests, test_tcl, test_tk, test_ttk_guionly, and test_ttk_textonly alphabetically follow test_idle, they should not be the cause of the crash during test_idle.  On the other hand, if the test above crashes, and so does 
xvbf ... -ugui test___all__ test_tcl test_tk test_ttk_guionly test_ttk_textonly
then test_idle would be eliminated as the direct culprit.