cases_generator tests aren't run by CI, and are broken

[gvanrossum]

There are some tests for the cases generator in Tools/cases_generator/test_generator.py, but several things are wrong with these:

• They are currently broken
• They depend on pytest
• They aren't run in CI
• They are incomplete

Let's fix that (in that order).

Linked PRs

• gh-105540: Fix code generator tests #105707
• gh-105540: Convert pytest tests of cases_generator to regular tests #106713
• gh-105540: Show source files relative to root #106927
• gh-105540: Improve relative paths in generate_cases.py #106942

[gvanrossum]

This should be pretty easy -- just Python code. Especially the next step (removing the pytest dependency in favor of unittest) should be quite doable.