Resources extension: TemporaryDirectory: Support custom filesystem

[jbduncan]

Consider introducing the ability to change the filesystem that our TemporaryDirectory writes its temporary directories to. Currently it writes all directories to the local on-disk filesystem, but one may want to use jimfs or another filesystem instead to have more reliable tests or to be able to test unhappy paths more easily.

junit-pioneer used to have a TempDirectory class with this ability, but it was removed because the built-in JUnit 5 TempDir was superior in almost every way.

This issue is a follow-up to #348 | #491.

Relevant JUnit 5 issues:

• Support custom file system for @TempDir junit-team/junit5#2400

Docs for the old TempDirectory extension: https://github.com/junit-pioneer/junit-pioneer/blob/1415785e909f82066df2bd2b5ccfa10396aab0a0/docs/temp-directory.adoc

[scordio]

Hey Pioneer team, are you still willing to tackle junit-team/junit5#2400?

I'm also interested in that feature due to assertj/assertj#2567 so I'd be happy to give it a try, but you were in line first 😉

[nipafx]

It has been a long time in the making, but #491 should be merged soon-ish. At that point it should be really easy to provide a custom file system as temporary resource. If you're particularly enthusiastic, you can start off of that PR (chances are very high that the code won't be changed at all or at least not much before the merge) and already work on that extension.