servertest, integration: cover HA both-offline recovery

Three regression tests for the user scenario: an in-process
Disconnect/Reconnect, a tailscale-down/up integration test, and
an iptables -j DROP cable-pull integration test.

Updates #3203

.github/workflows/test-integration.yaml

@@ -251,6 +251,8 @@ jobs:
           - TestSubnetRouteACLFiltering
           - TestGrantViaSubnetSteering
           - TestHASubnetRouterPingFailover
+          - TestHASubnetRouterFailoverBothOffline
+          - TestHASubnetRouterFailoverBothOfflineCablePull
           - TestHeadscale
           - TestTailscaleNodesJoiningHeadcale
           - TestSSHOneUserToAll