Add a test for any function

[e-kotov]

Choose any function in the package and write a test for it using {testthat}. If this function requires internet access (and ultimately, it seems like most of them do) make sure this test is skipped on cran and probably in CI/CD too.

As a good example for how to skip the tests that require downloading some data, you can check the tests in {opentripplanner} package here and also in {r5r} here.