Make sure there are sanity tests for the javatools_backend project that are run as part of the GitHub workflow pipeline

[lagergren]

To make sure that javatools_backend continues to resolve, builds and run on all platform, we should add a GitHub workflow test before merge to master.

The template at ".github/workflows/xvm-verify-push.yml" is trivial to modify to add sanity checks for building, compiling and running XTC modules. Use any of the available GitHub actions, or write something manual. It's enough that a test task exits with a non-zero OS exit code on failure (and 0 for success), for it to serve as push verification.

(Extra ergonomic tip re testing GitHub workflows: The "act" app is an out-of-the-box solution that enables you to run all github workflows on your local machine, for testing purposes. That way you don't need to push to your branch, when you change something in a workflow under development. It is ergonomically very nice to not have to have a 30 second turnaround for discovering a typo)