fake_filesystem makes debugging extremly hard

Recently, the fake_filesystem was introduced for testing. While debugging some changes to `root_inc_files` (see #468 and #506), I found it extremely difficult to diagnose what was going wrong, since I couldn't just look at the file system. Running the tests in tmpdir makes that much easier.

But @MatthewHambley mentioned that there are debugging options for FakeFS (which I didn't find when googling). I am happy to stay with FakeFS if sufficient debugging options are available. Could you just add a link to what can be done (to check e.g. if a file 'exists' or has a certain content).

No documentation file, test, or entry point is named. Start by locating the project's documentation and the FakeFS debugging guidance, then add a link covering how to inspect virtual files and their contents; done means contributors can find those debugging options from the project documentation.

Written by the indexing model from the issue text.