HTTP auto tests: do not load resources from cache that must be revalidtd

HTTP auto tests: do not load resources from cache that must be revalidtd

The header field "Cache-Control: must-revalidate" is a strict
requirement for loading the resource from the server, and not reading it
from the cache without revalidating first. With this patch, PreferCache
will load such from the network instead of loading them from the cache,
and AlwaysCache will throw a ContentNotFound error.

Reviewed-by: Markus Goetz
Task-number: QTBUG-18983