PPPOE-HA: record the two failure modes found by running the thing

Both were invisible to inspection and only appeared under the hard matrix:
the flap damper tearing down an established session via the lease it stopped
renewing, and a missing peers file being completely silent. The second has a
production edge worth spelling out -- an unsaved commit reverts on reboot and
takes pppoe0 with it, leaving a standby that can never take over while
looking perfectly healthy.

labsim/labsim-pppoe-ha-test.sh

@@ -106,7 +106,10 @@ preflight() {
             fail "$h has no /etc/ppp/peers/pppoe0 -- it cannot dial; re-commit the pppoe subtree"
             rc=1
         fi
-        if ! r "$h" 'show configuration commands' 2>/dev/null | grep -q 'interfaces pppoe pppoe0 source-interface'; then
+        # The op-mode WRAPPER, not a bare `show`: over non-interactive ssh the
+        # bare form is not on PATH, so this silently matched nothing and failed
+        # both routers that were in fact configured correctly.
+        if ! r "$h" '/opt/vyatta/bin/vyatta-op-cmd-wrapper show configuration commands' 2>/dev/null | grep -q 'interfaces pppoe pppoe0 source-interface'; then
             fail "$h has no pppoe0 in config (unsaved commit lost on reboot?)"
             rc=1
         fi