Empty test to check default request path

... is served from Origin by default.
Empty test to describe behaviour when no backend

We currently get a 'Guru Meditation Error' from varnish
when we do not have a valid backend to serve the request
from. Document this behaviour in a test, with a note to
improve with a consistent page.
Empty tests describing useful response headers

These response headers are set by Fastly, and have been
useful in debugging and analysis. They are 'nice to haves' from
any replacement provider, and serve as a note that we could implement
these.