Generate CA Certificates that are valid for a bit further back in time (bug #1352944).

See bug #1352944. We were generating certificates that were valid 5 minutes ago, to avoid
problems with the clock on the client being out of sync with the clock on the server, but
it seems 5 minutes isn't quite enough to account for real world clock skew. So bump it
up to 1 week.
I'd like to see a comment what we're testing here (i.e. making sure the cert is valid for 1 week before "now").