Bound the VAX image test's wait for sshd on the clock

[jacob-carlborg]

The wait counted attempts — 180 of them, spaced by sleep 20 — and the comment claimed that came to about an hour:

ok=
for _ in $(seq 1 180); do
  if ssh_vax true 2>/dev/null; then ok=1; break; fi
  sleep 20
done

It only comes to an hour when every attempt fails instantly, which is what happens while the guest's ssh port is still closed: slirp answers with an RST. An attempt against a guest that accepts the connection and then stalls costs the whole ConnectTimeout instead — 60s here — making the same loop nearly four hours.

That is not hypothetical. Two VAX jobs on the same commit failed the same nominal way and took wildly different amounts of time:

job	outcome	Test Image (vax)
10.1, vax	failed	60m21s — every attempt RST, so the intended bound held
11.0, vax	still running when noticed	2h+, and would have run to ~4h

So the bound depended on how the guest was broken, which is the one thing a timeout should not depend on.

The fix

Take the deadline off the clock, so it holds regardless of what an individual attempt costs:

deadline=$(($(date +%s) + 3600))
ok=
while [ "$(date +%s)" -lt "$deadline" ]; do
  if ssh_vax true 2>/dev/null; then ok=1; break; fi
  sleep 20
done

The overshoot is now just the attempt in flight when the hour passes, instead of a multiple of the whole budget. Verified against a stubbed ssh_vax at 1/1000th scale — exits immediately on success, and honours the budget whether attempts fail instantly or burn time (overshooting by one attempt, ~80s at real scale).

The comment also loses its "~15 minutes" figure. A healthy guest reaches sshd in about two minutes; that number described first-boot host key generation, which the image stopped doing when the keys started being baked in.

Scope

Branched from master, and touches only the VAX test step, so it is independent of both #11 and the ntpdate fix in #12. This is purely about the harness bounding itself — #12 addressed one cause of a stalled guest, whereas this makes any future cause cost an hour instead of four.

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