Use regular install for CI tests

[jfrost-mo]

The CI tests are now installed with a regular pip install, rather than being installed editabally. This ensures they are more realistic.

• Investigate why the coverage has dropped because of this. (It seems the CLI tests are being missed.)

[github-actions]

[jfrost-mo]

Probably the thing to do is to ditch the end-to-end CI tests, just keeping the --help one for validating that the cset command still works, and instead test the code inside __init__.py more explicitly.

This will probably involve marking the if __name__ == "__main__" bit as noqa, as I don't thing we can measure coverage on those couple lines.

[jfrost-mo]

This causes the test coverage to drop.