Provide a set of default transformers for snapshot tests?

[kevinushey]

Motivated by what we've added in renv, it feels like some subset could be useful more broadly in testthat. (The primary motivation: it can be frustrating when you try to write a snapshot test only to find it fails on Windows on CI because of something OS-specific.)

Some options:

• R version,
• Package version(s),
• Working directory (including aliased paths?)
• R temporary directory (including aliased paths?)
• Repository paths,
• Library paths,
• Elapsed times?

[hadley]

And it would be convenient if transformer could take a list of functions.

[hadley]

I haven't felt the itch for the helpers I think it's probably better to create on a case-by-case basis.