title: Where to put external files for testthat tests
body: <p>Suppose I have test like this:</p>
  
  <pre><code>require(testthat)
  context("toy test")
  test_that("toy", {
              df = my.read.file("test.txt", header=TRUE)
              expect_true(myfunc(df) == 3.14)
  })
  </code></pre>
  
  <p>and this test relies on a external file test.txt, where should I put this file then?</p>