capture output of tests on windows to "dev/null"

[matteodelucchi]

the pendent of "/dev/null" on windows is "nul".
https://stackoverflow.com/questions/4507312/how-to-redirect-stderr-to-null-in-cmd-exe

Currently, when the output of tests is captured in "/dev/null" the tests are omitted on windows.
Consider instead sth like this:

test_that("plot.abnDag() works.", {
  mydag <- createAbnDag(dag = ~a+b|a, data.df = data.frame("a"=1, "b"=1))

  if(.Platform$OS.type == "unix") {
    FILE <- "/dev/null"
  } else {
    FILE <- "nul"
  }
  capture.output({
    expect_no_error({
      plot(mydag)
      })
    },
    file = FILE)
})

Not sure this example works well...