gh-106368: Improve coverage reports for argument clinic

[AlexWaygood]

• Use ... rather than pass for empty bodies in abstract methods that are never meant to be executed. Exclude lines consisting solely of ... in our .coveragerc file.
• Use raise AssertionError instead of raise RuntimeError for an exception that's never meant to be executed -- it would indicate a bug in Tools/clinic/ itself, rather than an incorrect usage of argument clinic, if that line was ever executed. Exclude raise AssertionError( lines in our .coveragerc file.
• A reasonably large block of code in Tools/clinic/cpp.py was being excluded from coverage reports because it was all inside an if __name__ == "__main__" block, and if __name__ == "__main__" blocks are excluded in our .coveragerc file. That was making our coverage for cpp.py look better than it actually was. Move that block inside a _main() function so that it's correctly reported as being uncovered in coverage reports.

• Issue: Increase Argument Clinic test coverage #106368

[erlend-aasland]

Thanks!