ipn/ipnlocal: fail test if more notifies are put than expected

The `put` callback runs on a different goroutine to the test, so calling
t.Fatalf in put had no effect. `drain` is always called when checking what
was put and is called from the test goroutine, so that's a good place to
fail the test if the channel was too full.

Updates #17363

Signed-off-by: James Sanderson <jsanderson@tailscale.com>

ipn/ipnlocal/state_test.go

@@ -59,8 +59,9 @@ type notifyThrottler struct {
 
 	// ch gets replaced frequently. Lock the mutex before getting or
 	// setting it, but not while waiting on it.
-	mu sync.Mutex
+	mu     sync.Mutex
-	ch chan ipn.Notify
+	ch     chan ipn.Notify
+	putErr error // set by put if the channel is full
 }
 
 // expect tells the throttler to expect count upcoming notifications.
@@ -81,7 +82,11 @@ func (nt *notifyThrottler) put(n ipn.Notify) {
 	case ch <- n:
 		return
 	default:
-		nt.t.Fatalf("put: channel full: %v", n)
+		err := fmt.Errorf("put: channel full: %v", n)
+		nt.t.Log(err)
+		nt.mu.Lock()
+		nt.putErr = err
+		nt.mu.Unlock()
 	}
 }
 
@@ -91,8 +96,13 @@ func (nt *notifyThrottler) drain(count int) []ipn.Notify {
 	nt.t.Helper()
 	nt.mu.Lock()
 	ch := nt.ch
+	putErr := nt.putErr
 	nt.mu.Unlock()
 
+	if putErr != nil {
+		nt.t.Fatalf("drain: previous call to put errored: %s", putErr)
+	}
+
 	nn := []ipn.Notify{}
 	for i := range count {
 		select {