test_idle, which runs the tests in idlelib/idle_test, currently comprises 9 test methods and perhaps 100 lines that do not, as far as I know, use warnings or logging. However, perhaps this has an effect:

F:\Python\dev\cpython\PCbuild>python_d -m test test_idle
[1/1] test_idle
Warning -- os.environ was modified by test_idle
Warning -- locale was modified by test_idle
1 test altered the execution environment:
    test_idle

Of course, the idle test code itself does no such modification. So I have no idea what the warning is about or how to stop it.

F:\Python\dev\cpython\PCbuild>python_d -m test.test_idle
which runs under unittest, does not produce the warning.

The other test files that use tcl/tk/tkinter, test_tcl, test_tk, test_ttk_guionly, and test_ttk_textonly, produce the os.environ warning but not the locale warning.

I duplicated the failure with the command you gave. Substituting _tcl or _tk for _idle removes the error. So perhaps the locale alteration, however it happens, is the problem.