Previews - folder visibility intermittent test failure

The test utility purges the folder content library and then triggers a rebuild.
However if the PP is running, a rebuild will (eventually) also be triggered which
intermittently cause issues. To avoid this, the test utility should not be used
in the preview processor tests
Merge branch 'preview-intermittent-failure' of https://github.com/simong/Hilary into preview-intermittent-failure