Since we are using tmpdirs in order to enable parallel testing, we
can go the full hog and drop the fakefs too and just copy all the
files directly into the tmpdir. If a test makes changes (e.g.
changing the icon file during an import) the copy in the tmpdir is
thrown away at the end of the test anyway.
The mocking and stubbing in the controller tests is
unfortunate, but the models interact directly with the filesystem
using the trace id so that's hard to control any other way.
