Idiomatic test for file/directory creation

Question

Say I have a function that creates a directory and file:

foo <- function(){
  dir.create("bar")
  file.create("./bar/myfile.txt")
}

I need to test that the function does its job. I can do this with expect_true from the testthat package, like so:

test_that("File & directory are created",{
  foo()
  expect_true(file.exists("./bar/myfile.txt"))  
})

This seems to work okay, but my question is what is the idiomatic way of testing for file/directory creation using testthat? There are specific functions for other side effects (e.g., expect_output), but I haven't found an equivalent for files/directories.