Many tests require some external file (e.g. a .csv if you're testing a
data import function) but the working directory varies depending on the way
that you're running the test (e.g. interactively, with devtools::test(),
or with R CMD check). test_path() understands these variations and
automatically generates a path relative to tests/testthat, regardless of
where that directory might reside relative to the current working directory.