Using vdiffr unconditionally in package tests

[gavinsimpson]

Brian Ripley writes

Suggested packages should be used conditionally: see §1.1.3.1 of
'Writing R Extensions'. Some of the requirements of vdiffr are hard to
install on a platform without X11 such as M1 Macs: see the logs at
https://www.stats.ox.ac.uk/pub/bdr/M1mac/.

Lionel Harvey suggested the solution:

A simple way to make vdiffr usage conditional is to define your own
expect_doppelganger() as follows:

expect_doppelganger <- function(title, fig, path = NULL, ...) {
  testthat::skip_if_not_installed("vdiffr")
  vdiffr::expect_doppelganger(title, fig, path = path, ...)
}

You then call expect_doppelganger() without the vdiffr:: prefix.
See lionel-/ggstance@eac216f6.

Fix by Jan 12, 2021