By restricting the Date field to be in the past, an attacker cannot
just create a repository from the future that would be accepted as
a valid update for a repository.
This check can be disabled by Acquire::Check-Date set to false. This
will also disable Check-Valid-Until and any future date related checking,
if any - the option means: "my computers date cannot be trusted."
Modify the tests to allow repositories to be up to 10 hours in the
future, so we can keep using hours there to simulate time changes.