Tests no longer connect to test servers they closed (closes #257)

Four tests closed a test server or stopped discriminator, and then
connected to its port. Another test process could take that port in
between and receive the connection.

- TestUnreachableRouteWatchLetsTheRequestThroughAndSaysWhy and
  TestUnreachableRoutewatchFailsTheQueryAndTheLogOmitsTheClient, formerly
  TestRefusedConnection..., point routewatch at 224.0.0.1, a multicast
  address to which TCP cannot connect.
- TestStalledRemoteLogEndpointHoldsUpNoRequest keeps its endpoint open
  and closes each connection it takes, instead of closing the endpoint.
- TestTheSiteAnswersWhileDiscriminatorIsStoppedAndStartedAgain gives
  rproxy an address the test holds throughout. The test forwards each
  connection there to the running discriminator, and closes it while
  discriminator is stopped.

No change to non-test code.

Model: opus-5-5

internal/sww/rproxy_test.go

@@ -280,7 +280,7 @@ func startForwarder(t *testing.T) (string, func(addr string)) {
 	})
 
 	var (
-		lock sync.Mutex
+		mu sync.Mutex
 		to string
 	)
 
@@ -288,8 +288,8 @@ func startForwarder(t *testing.T) (string, func(addr string)) {
 	// holds the lock throughout, so once another address is set, nothing
 	// connects to the one before.
 	dial := func() net.Conn {
-		lock.Lock()
-		defer lock.Unlock()
+		mu.Lock()
+		defer mu.Unlock()
 
 		if to == "" {
 			return nil
@@ -323,8 +323,8 @@ func startForwarder(t *testing.T) (string, func(addr string)) {
 	}()
 
 	return listener.Addr().String(), func(addr string) {
-		lock.Lock()
-		defer lock.Unlock()
+		mu.Lock()
+		defer mu.Unlock()
 
 		to = addr
 	}