The essential problem in both this issue and issue 30672 is that the tests are currently incorporating some Linux-specific assumptions about ways to request the "C" locale.

In https://github.com/python/cpython/pull/4369, I've taken the approach of making the baseline tests only cover "C" and "invalid.ascii", and then explicitly *opt-in* to testing an empty locale and "POSIX" on Linux machines.

If that's enough to get the test passing on Cygwin, I'm inclined to leave it at that. Dynamically calculated test expectations always make me nervous, since it's all too easy to end up with bugs that impact both the test case and the expectation calculator in the same way, and hence end up with the test passing when it should really fail.