Before this change TestPurge would remove a container and subsequent
tests would fail because the container was still being deleted so
couldn't be created.
This was fixed by introducing an fstest.NewRunIndividual() test runner
for TestPurge which causes the test to be run on a new container.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
