wgengine/bench: handle multiple Engine status callbacks

It is possible to get multiple status callbacks from an Engine.
We need to wait for at least one from each Engine.
Without limiting to one per Engine,
wait.Wait can exit early or can panic due to a negative counter.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/bench/wg.go

@@ -87,6 +87,7 @@ func setupWGTest(b *testing.B, logf logger.Logf, traf *TrafficGen, a1, a2 netadd
 	var wait sync.WaitGroup
 	wait.Add(2)
 
+	var e1waitDoneOnce sync.Once
 	e1.SetStatusCallback(func(st *wgengine.Status, err error) {
 		if err != nil {
 			log.Fatalf("e1 status err: %v", err)
@@ -118,9 +119,10 @@ func setupWGTest(b *testing.B, logf logger.Logf, traf *TrafficGen, a1, a2 netadd
 		}
 		c2.Peers = []wgcfg.Peer{p}
 		e2.Reconfig(&c2, &router.Config{}, new(dns.Config))
-		wait.Done()
+		e1waitDoneOnce.Do(wait.Done)
 	})
 
+	var e2waitDoneOnce sync.Once
 	e2.SetStatusCallback(func(st *wgengine.Status, err error) {
 		if err != nil {
 			log.Fatalf("e2 status err: %v", err)
@@ -152,7 +154,7 @@ func setupWGTest(b *testing.B, logf logger.Logf, traf *TrafficGen, a1, a2 netadd
 		}
 		c1.Peers = []wgcfg.Peer{p}
 		e1.Reconfig(&c1, &router.Config{}, new(dns.Config))
-		wait.Done()
+		e2waitDoneOnce.Do(wait.Done)
 	})
 
 	// Not using DERP in this test (for now?).