Make Age test less prone to backend cache sync

... and make our tests more informative when it still fails
Fix Age header test to reflect Varnish behaviour

We have made an interesting discovery during the writing of this
test.

Varnish appears to have slightly different behaviour with respect to
the Age header - at least between the Fastly version and our 3.0.5
mock test deployment.

In Varnish 3.0.5, the Age header would be increased based on the
value provided by origin -- that is to say an origin Age of 100 would
appear to be Age 100 when it went into the cache, and 105 5s later.

In Fastly Varnish, we were seeing this Age header get reset to 0, but
only in this test. In production, the former behaviour was noted.

@dcarley discovered that if the Cache-Control header was also set with
a max-age value set, the production behaviour was replicated.

This part of the Varnish source appears to relate to this functionality:

https://github.com/varnish/Varnish-Cache/blob/varnish-3.0.5/bin/varnishd/rfc2616.c#L42
Highlight need to prevent CDN location switch