Fix zone-to-zone (XDE test, kbench local) runner

During setup, we are not doing any work to ensure that Helios has a
valid NDP cache entry ready to use over the simnet link we install for
testing. As a result, XDE selects the right output port, but installs
source and destination MAC addrs of zero.

This worked before; the devices were in promiscuous mode, so the packets
made into `xde_rx`. In other cases, the underlay traffic in e.g. a
SoftNPU deployment was priming all the necessary NCEs, so we always knew
the target MAC address. Obviously this is an easy fix here, but we also
need to ensure that all sleds have valid NDP entries for one another at
all times.

xde-tests/src/lib.rs

@@ -4,6 +4,7 @@
 
 // Copyright 2024 Oxide Computer Company
 
+use anyhow::Context;
 use anyhow::Result;
 use opteadm::OpteAdm;
 use oxide_vpc::api::AddRouterEntryReq;
@@ -349,6 +350,27 @@ pub fn two_node_topology() -> Result<Topology> {
     println!("setup zone b");
     b.setup(&vopte1.name, opte1.ip())?;
 
+    // We now need to establish an NDP cache entry both ways, otherwise
+    // we'll write zero for both MAC addrs and the packet *will* be dropped --
+    // we're not in promisc anymore :). One ping from the global zone will suffice.
+    let ping_res = Command::new("ping")
+        .args([
+            "-A",
+            "inet6",
+            "-i",
+            &sim.end_b,
+            &format!("{}%{}", ll0.ip, sim.end_a),
+        ])
+        .output()
+        .with_context(|| "calling 'ping' over simnet")?;
+
+    if !ping_res.status.success() {
+        anyhow::bail!(
+            "Failed to ping over simnet links!\nstderr:{:?}",
+            std::str::from_utf8(&ping_res.stderr)
+        );
+    }
+
     Ok(Topology {
         xde,
         lls: vec![ll0, ll1],