I just tried out http://www.bitvise.com/winsshd , which is free for personal
and noncommercial use. If you run the tests via an ssh connection, WinSSHD
automatically translates pop-ups into error messages:

C:\Users\stefan\cdecimal\PCbuild>python.exe -m test -uall test_capi
[1/1] test_capi
Warning -- threading._dangling was modified by test_capi
test test_capi failed -- Traceback (most recent call last):
  File "C:\Users\stefan\cdecimal\lib\test\test_capi.py", line 51, in test_no_FatalError_infinite_loop
    b'Fatal Python error:'
AssertionError: b"Fatal Python error: PyThreadState_Get: no current thread\n\r\nThis application has requested the Runtime to terminate it in an unusual way.\nPlease contact the application's support team for more information." != b'Fatal Python error: PyThreadState_Get: no current thread'

1 test failed:
    test_capi                              



Thus, by using ...

   self.assertIn(b'Fatal Python error:', err.rstrip())

in test_capi, the tests pass without any further intervention! This is most
interesting, to the point that I'm considering setting up a buildbot.