When initially creating the GitHub Actions
workflow, it was obviously forgotten to make
it also run on pull requests, not only when
changes are pushed to branches in the
original repository (not a fork).

As the hottest of the press features of GitHub Actions
requires using YAML rather than HCL.

While creating the `$ npm install` step in the
GitHub Actions workflow, I forgot to state the fact that
the `$ npm test` step "needs" the install deps step
to have successfully run first.

..forgot to install dependencies before executing
`$ npm test` which obviously exploded due to the
fact that `eslint` and friends weren't available.

As a means of seeing how GitHub Actions
work compared to Travis CI, these changes creates
a so-called workflow that runs `$ npm test`
upon every `$ git push` to the repository.