Set option when snapshotting

[hadley]

Fixes #1796

[jennybc]

I don't necessarily think this matters but looking at with_ functions I have implemented, I seem to do it like this:

with_is_snapshotting <- function(code) {
  withr::with_options(list(testthat_is_snapshotting = TRUE), code = code)
}

I guess maybe the one difference is whether you force(code) or not?

[hadley]

I prefer the local_ syntax pretty much everywhere.

[kevinushey]

LGTM, although I'm curious what the convention is between testthat.option versus testthat_option naming is.

[hadley]

@kevinushey we're trying to move towards _ everywhere, but there's a lot of places that . remain.