test/integration: wall-clock SSH wait with progress

wait_for_ssh counted only its sleeps toward the timeout. Under QEMU user
networking each attempt can also spend its whole ConnectTimeout waiting
for a banner, so a "300 s" wait ran for ten minutes or more without a
line of output. It now uses a wall-clock deadline, polls every 2 s, and
prints a progress line with a console screenshot every 30 s. When the
boot scenario times out it prints the serial console's last lines too.

test/integration.d/boot-test.sh

@@ -12,9 +12,15 @@ base_image_ready || { echo "No base image; run this through ./test/integration"
 log "Booting the installed system"
 boot_started=$(date +%s%N)
 start_vm_from_base
-wait_for_ssh 300 failure-boot-ssh-timeout || exit 1
+if ! wait_for_ssh 300 failure-boot-ssh-timeout; then
+  # The console screenshot is saved; the serial log shows what the firmware,
+  # loader and kernel printed, which a screenshot of a hung boot often lacks.
+  echo "--- last lines of the serial console ---" >&2
+  tail -n 40 "$RUN_DIR/serial.log" >&2 2>/dev/null || true
+  exit 1
+fi
 boot_ms=$((($(date +%s%N) - boot_started) / 1000000))
-# Coarse: wait_for_ssh polls every 5 s. systemd-analyze below has the exact
+# Coarse: wait_for_ssh polls every 2 s. systemd-analyze below has the exact
 # firmware, loader, kernel and userspace split.
 log "SSH answered within ${boot_ms} ms of power-on"