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.