Tighten serve_stale dig timeouts and inter-step sleeps

Tighten serve_stale dig timeouts and inter-step sleeps

With the TCP fallback now actually firing after repeated UDP timeouts,
the resolver covers more retry transitions in the same wall-clock
window, and the original 3-second budgets in two steps of the
serve_stale test left no margin: the dig client at +timeout=3 and the
"sleep 3" before re-enabling the upstream both straddled the moment at
which the resolver switched transport, making the asserted outcome
race-prone.

Drop the dig timeout to 2s and the sleep to 1s so each step lands
firmly on one side of the transport switch.

Co-authored-by: Evan Hunt <each@isc.org>
Assisted-by: Claude:claude-opus-4-7