caddy-frontend: Stabilise negative revalidation

Thanks to work on TrafficServer bug 7425 it was possible to understand
correctly the switches and setup ATS to SlapOS CDN needs.

max_stale_age with negative_revalidating_enabled is used to simulate
stale-if-error and negative_revalidating_lifetime is set to 0, as Expires
header provided by this switch is needless in SlapOS case.

Tests are updated to follow the real use case, by rewriting ATS configuration
to meet times required for the test to finish in reasonable time.