Suggesting vdiffr but not using it conditionally

[xrobin]

Received an email from CRAN about vdiffr not being properly conditional in tests.

Followup from Lionel Henry:

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

This needs to be fixed until January 12, 2021.

[xrobin]

Works with _R_CHECK_DEPENDS_ONLY_=true.
Now only needs to be pushed to CRAN.