I think it is actually pretty straightforward to write a *unit* test for this.  We just need to check that the logic works correctly given the expected presence or absence of the environment variables.  That doesn't test whether or not the right thing happens in the environment when you actually run a WOW64, but I don't think it is Python's responsibility to test that.  If Microsoft changes the API, platform will break and the tests won't notice, but I don't think there's anything we can do about that, since as you say the API is the only way to find out what to expect for results.

Test patch attached.  Brian, if you can confirm that this test fails before your patch and succeeds afterward, I will commit both patches.