Internals: Avoid deletion of unused snapshots when running tests if {doconv} is not installed

[markheckmann]

Currently, if {doconv} or MS Office / LibreOffice is not installed on the system when running the tests, {testthat} will remove unused snapshots. As discussed here, this appears to be normal {testhat} behavior (also a workaround is supplied). It's nothing critical but could be fixed just for convenience.

[davidgohel]

I'd like to totally get rid of these visual tests or reduce them to a resulting minimum set of snapshots. Since officer is quite stable now, I think it's possible. I hope I'll have time for it soon.

'doconv' is useful but not very reliable because Office 365 check the license and maybe other operations that make it quite slow. Also it can't work in Github Actions.