PR tests should fail if generate_schema_wrapper produces changes

[binste]

Altair has many files which are auto-generated based on the Vega-Lite schema. For development, it is often convenient to modify those directly but before merging into master, these changes need to be applied to the relevant scripts in the tools folder which is a step that is easy to forget.

It would be great if the tests for a PR fail if changes were applied to auto-generated files. One way could be to add a new step in build.yml which runs python tools/generate_schema_wrapper.py and then checks if any files were changed with git ls-files --modified.

[joelostblom]

This sounds like a good idea and will make it easier to keep track of which files need changes.

[binste]

Great, I will create a PR for this.