Add GitHub Actions test workflow

[jelovirt]

It seems the project no longer uses Travis. This PR adds a test GitHub Actions workflow that

1. Generates module build scripts
2. Builds JAR file
3. Runs validation target
4. Builds Jing distribution
5. Builds Trang distribution
6. Builds DTDinst distribution
7. Builds website

While just running the dist target would run all the preceding targets, running them separately makes it easier to track at which point the build has failed.

See https://github.com/jelovirt/jing-trang/actions/workflows/test.yml to see how workflow looks in GitHub UI.