Having run some more tests, it may be that the only regular problem is in the test for inherited file descriptors.

I've attached a patch for tf_inherit_check.py that will prevent assert dialogs. It's not pretty, but it avoids touching the interpreter internals. I still prefer the environment variable option (and maybe add an underscore at the start/making it 'officially' undocumented?), as that will help avoid issues when new tests are added, but for now it looks like only this test needs fixing.