> In the meanwhile, I will try using support.import_fresh_module('warnings') in test_logging, or better, saving and restoring warnings in test_idle.

I agree the second option (changing test_idle) is better. The failing logging test is checking to ensure that the original warnings implementation is used when an explicit file is passed to warnings.showwarning, but it assumes that original implementation is the one in warnings when comparing the output from that (supposed) original implementation.

+1 to restoring the environment and locale in test_idle, too.