Use vdiffr conditionally

[yutannihilation]

I also got this email as I'm the maintainer of a package that uses vdiffr. It says we need to fix this until 2021-01-12. On master, the use of vdiffr is already made conditional by #4192, so it's just patching this commit to the released version.

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.

@thomasp85 @hadley
Are you already planning a patch release for this? Please let me know if there's anything I can help.

[hadley]

@thomasp85 is out sick for the next few days, but we'll make this a priority when he's back.

[yutannihilation]

Oh, take care. I see. We have 1 month and this is easy to fix, I don't worry so much.

[yutannihilation]

It seems we also need to tweak some tests to address the change of all.equal() on R-devel.

#4294