Add in_snapshotter() and return conditions visibly when snapshotting

[DavisVaughan]

This is currently broken, because in_snapshotter() seems to always return TRUE when testing a file or directory, and always return FALSE when running a test interactively. Both of these are incorrect.

We need something that is more fine grained. I think expect_snapshot() itself might need to set something that announces that you are "in a snapshot expectation". That would mean that both interactive and non-interactive testing could return visible conditions where appropriate.

[hadley]

I'm now having second thoughts about conditionally changing the visibility depending on whether or not you're in a snapshot or not. It just feels a bit too magical. What do you think?

[hadley]

Part of #1471

[DavisVaughan]

If we need to continue wrapping expect_error() in another set of parenthesis to avoid this feeling too magical, that is okay with me.

I would like to avoid seeing the err() wrapper in too many places, I'd rather be explicit about the (expect_error()) pattern
(mentioned in #1471 (comment))