tstest/integration: add some debugging for TestAddPingRequest flakes

This fails pretty reliably with a lot of output now showing what's
happening:

TS_DEBUG_MAP=1 go test --failfast -v -run=Ping -race -count=20 ./tstest/integration --verbose-tailscaled

I haven't dug into the details yet, though.

Updates #2079

tstest/integration/testcontrol/testcontrol.go

@@ -7,6 +7,7 @@ package testcontrol
 
 import (
 	"bytes"
+	"context"
 	crand "crypto/rand"
 	"encoding/binary"
 	"encoding/json"
@@ -46,6 +47,7 @@ type Server struct {
 	mux         *http.ServeMux
 
 	mu            sync.Mutex
+	cond          *sync.Cond // lazily initialized by condLocked
 	pubKey        wgkey.Key
 	privKey       wgkey.Private
 	nodes         map[tailcfg.NodeKey]*tailcfg.Node
@@ -68,6 +70,47 @@ func (s *Server) NumNodes() int {
 	return len(s.nodes)
 }
 
+// condLocked lazily initializes and returns s.cond.
+// s.mu must be held.
+func (s *Server) condLocked() *sync.Cond {
+	if s.cond == nil {
+		s.cond = sync.NewCond(&s.mu)
+	}
+	return s.cond
+}
+
+// AwaitNodeInMapRequest waits for node k to be stuck in a map poll.
+// It returns an error if and only if the context is done first.
+func (s *Server) AwaitNodeInMapRequest(ctx context.Context, k tailcfg.NodeKey) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	cond := s.condLocked()
+
+	done := make(chan struct{})
+	defer close(done)
+	go func() {
+		select {
+		case <-done:
+		case <-ctx.Done():
+			cond.Broadcast()
+		}
+	}()
+
+	for {
+		node := s.nodeLocked(k)
+		if node == nil {
+			return errors.New("unknown node key")
+		}
+		if _, ok := s.updates[node.ID]; ok {
+			return nil
+		}
+		cond.Wait()
+		if err := ctx.Err(); err != nil {
+			return err
+		}
+	}
+}
+
 // AddPingRequest sends the ping pr to nodeKeyDst. It reports whether it did so. That is,
 // it reports whether nodeKeyDst was connected.
 func (s *Server) AddPingRequest(nodeKeyDst tailcfg.NodeKey, pr *tailcfg.PingRequest) bool {
@@ -85,8 +128,7 @@ func (s *Server) AddPingRequest(nodeKeyDst tailcfg.NodeKey, pr *tailcfg.PingRequ
 	s.pingReqsToAdd[nodeKeyDst] = pr
 	nodeID := node.ID
 	oldUpdatesCh := s.updates[nodeID]
-	sendUpdate(oldUpdatesCh, updateDebugInjection)
-	return true
+	return sendUpdate(oldUpdatesCh, updateDebugInjection)
 }
 
 type AuthPath struct {
@@ -414,17 +456,19 @@ func (s *Server) updateLocked(source string, peers []tailcfg.NodeID) {
 }
 
 // sendUpdate sends updateType to dst if dst is non-nil and
-// has capacity.
-func sendUpdate(dst chan<- updateType, updateType updateType) {
+// has capacity. It reports whether a value was sent.
+func sendUpdate(dst chan<- updateType, updateType updateType) bool {
 	if dst == nil {
-		return
+		return false
 	}
 	// The dst channel has a buffer size of 1.
 	// If we fail to insert an update into the buffer that
 	// means there is already an update pending.
 	select {
 	case dst <- updateType:
+		return true
 	default:
+		return false
 	}
 }
 
@@ -489,6 +533,7 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey tailcfg.M
 		sendUpdate(oldUpdatesCh, updateSelfChanged)
 	}
 	s.updateLocked("serveMap", peersToUpdate)
+	s.condLocked().Broadcast()
 	s.mu.Unlock()
 
 	// ReadOnly implies no streaming, as it doesn't