Test helpers for working with files and folders

[martonvago]

Have a think about whether there are any useful common functions for testing that would help us to set up (and tear down) folder structures quickly when we run tests against them.

This is probably best done after we've written a couple of tests and see what our needs actually are.

[lwjohnst86]

Make use of temporary paths with tempfile module: tempfile.TemporaryFile() etc. See https://docs.python.org/3/library/tempfile.html

[lwjohnst86]

Not sure if this is relevant anymore?

[signekb]

I guess this comes up naturally as we create tests? For example, I added some helpers for testing the path_*() functions in #730
As we go, we can keep in mind how we want to structure these helper functions and fixtures?