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/discriminator/routewatchtags_test.go

@@ -153,14 +153,10 @@ func TestUnsetRouteWatchChangesNothing(t *testing.T) {
 func TestUnreachableRouteWatchLetsTheRequestThroughAndSaysWhy(t *testing.T) {
 	t.Parallel()
 
-	// A stand-in that is closed at once, so that the port refuses connections.
-	dead := httptest.NewServer(http.NotFoundHandler())
-	url := dead.URL
-
-	dead.Close()
-
+	// Another test can take a closed server's port at once, so routewatch's
+	// address here is a multicast address, to which TCP cannot connect.
 	s, _, _ := startWithClock(t, "", map[string]string{
-		lookupSource: off, routeWatchURL: url,
+		lookupSource: off, routeWatchURL: "http://224.0.0.1",
 	})
 
 	wantTagsMissing(t, s.get(fromDE, http.StatusOK, requestlog.ActionForward),