This is a single test for RstripExtension.py, following from #15392. I also added a mock EditorWindow module with a Text widget for testing. test_rstripextension.py seems to run fine inside IDLE and produces the following output:

>>> 
test_do_rstrip (__main__.Test_rstripextension) ... ok

----------------------------------------------------------------------
Ran 1 test in 0.100s

OK

However, when I run via the command line, I get the following output:

philwebster@ubuntu:~/Dev/cpython$ ./python -m test -ugui test_idle
[1/1] test_idle
Warning -- warnings.showwarning was modified by test_idle
1 test altered the execution environment:
    test_idle

I attempted to replicate the results from #18189, but saw the same message. Any thoughts?