[TEST] Add automatic tests #6

Also update the GitHub workflow introduced by #25
See https://r-pkgs.org/tests.html and https://r-pkgs.org/r-cmd-check.html#tests-1

For coverage, see

• https://www.rostrum.blog/2020/08/09/ghactions-pkgs/#example-r2eng-package
• https://github.com/matt-dray/r2eng/blob/master/.github/workflows/test-coverage.yaml