sot is below vcl, but is a filters test, we can split bootstrapfixture
into a vcl needing bit and and non-vcl bit and filters test api
can be standalone and combined with whichever bit in order to form
pre and post vcl filters test
It turns out the constructors are all run back-to-back one per filter,
and we need to init and de-init in a sensible order to make multiple
tests that otherwise share the ContentBroker work properly.
