Implement tests

[nuest]

I've used testthat in the past, but I'd like to give tinytest with ttdo a try:

• https://cran.r-project.org/package=tinytest
• https://cran.r-project.org/package=ttdo

[nuest]

In #18 I try to activate both GitHub Actions and Travis CI, some resources:

• https://www.jimhester.com/talk/2020-rsc-github-actions/
• https://ropenscilabs.github.io/actions_sandbox/
• https://github.com/r-lib/actions/tree/master/examples

[nuest]

#18 has a few tests, which also run as GitHub actions:

More tests can be added later when more features are added.