Replace assertUnreachable with fail

[slozier]

assertUnreachable was removed in #561 and should be replaced with fail in tests.

[komodo472]

I'm trying to work on this issue and I'm wondering if using Tests/run.py is still the preferred way to run the python tests under Tests? If not, what is the preferred way to test this issue to see if I got it working?

[slozier]

Preferred way to run all tests is via pwsh make.ps1 test-all. You can also run a specific test with test-something. Although this one is kind of hard to test since by definition the code should be unreachable. 😄