integration: harden ACL test ergonomics

tsic.Curl returned ("", nil) when curl exited 0 with a zero-byte body —
the usual signature of a mid-stream reset — so EventuallyWithT could
not retry. Return an error on empty body instead.

Replace the 56 inline curl + assert.Len(13) blocks with
assertCurlDockerHostname so the empty-body fix benefits every callsite
without further touch-ups.

Gate ACL waits on actual filter visibility: snapshotClientFilters +
waitForClientFilterChange ensure the new PacketFilter has reached the
client before assertions fire; SyncOption + WithPreBarrier feeds a
server-side policy-loaded check into WaitForTailscaleSyncPerUser.

Move advertise-routes mutation out of EventuallyWithT in route_test
(cmd/hi/README forbids retrying state-mutating calls). Pace the
TestNodeOnlineStatus outer loop with a Ticker, not a Sleep.