use vdiffr conditionally

[simonpcouch]

From BDR to the vdiffr maintainers a few weeks ago:

...
> 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/.
>
> In some cases there are other suggested packages not used conditionally:
> you can check all of them by setting environment variable
> _R_CHECK_DEPENDS_ONLY_=true  -- see
> https://cran.r-project.org/doc/manuals/r-devel/R-ints.html#Tools .
>
> Please correct ASAP and before 2021-01-12 to safely retain the package
> on CRAN.

From the vdiffr maintainers:

Hi everyone,

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 https://github.com/lionel-/ggstance/commit/eac216f6.

Though vdiffr was in our Suggests:, we weren't previously using it conditionally. After these changes,

devtools::check(env_vars = c(NOT_CRAN = "true", `_R_CHECK_DEPENDS_ONLY_` = "true"))

comes back clean for me!

[echasnovski]

Looks great!
In my other package after much hesitation I decided to go with other wrapper name to avoid possible confusion, but this is also good. If Lionel recommends and uses this, we should be fine too :)

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