Version 14 that we used before is now in maintenance mode, so we should
upgrade to the most recent LTS version.
Moreover, use the most recent `setup-node` workflow version and syntax;
see https://github.com/actions/setup-node#usage.
This command was added all the way back when basic CI-support was first introduced (using Travis at the time), however it's never really intended to be used e.g. for local development.
By having a `npm test`-command listed in the `package.json` file, there's a very real risk that someone unfamiliar with the code-base would only run that one and thus miss all the other (more important) test-suites[1].
Hence this patch which removes the `npm test`-command, and instead simply calls the relevant gulp-task[2] directly in the GitHub Actions configuration.
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[1] Which consist of the unit-tests (run in browsers), the font-tests (potentially), the reference-tests, and the integration-tests.
[2] Which is also renamed slightly, to better fit its current usage.
This makes it consistent with the GitHub Advanced Security file and,
more importantly, ensures that all steps have a proper name for better
visibility.
