verify_output fails on Linux if there is no X11

[gaborcsardi]

In general it fails if there is on png() device.

TBH I am not sure if it is testthat's business to deal with graphic output at all, but at this point we might not be able to change this.

We could still use a more minimal device, e.g. the devoid package could be handy here?

[gaborcsardi]

I am not sure why but if there is a png() device, but there is no X11 support, it still fails. Probably because png() uses X11 by default, and will not fall back to cairo, even if there is no X11.

So this is my current workaround:

skip_without_png_device <- function() {
  if (.Platform$OS.type == "windows") return()
  if (! capabilities("png") || ! capabilities("X11")) {
    skip("Needs a PNG device")
  }
}