Don't use variant if only saving a single screenshot

[gadenbuie]

In several cases, in particular with focused tests that don't primarily rely on snapshots, we only take screenshots on a single OS and R version, e.g. MacOS + r-latest.

In these cases, we should set variant = NULL in the app options.

We should also have shared code that returns TRUE for mac-latest + r-latest, to facilitate deciding whether or not to take a screenshot.

As a side benefit, this also helps with the initial set up of a test; the first screenshots can be created locally regardless of OS and R version.