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/routewatch/routewatch_test.go

@@ -239,12 +239,17 @@ func TestAnswerRoutewatchCannotBeTakenFromFails(t *testing.T) {
 	}
 }
 
-func TestRefusedConnectionFailsTheQueryAndTheLogOmitsTheClient(t *testing.T) {
+func TestUnreachableRoutewatchFailsTheQueryAndTheLogOmitsTheClient(t *testing.T) {
 	t.Parallel()
 
 	f := newFixture(t, answering(routedAnswer))
 
-	f.server.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.
+	f.client = routewatch.New(routewatch.Params{
+		URL: "http://224.0.0.1/", Now: f.clk.Now,
+		ProcessLog: slog.New(slog.NewTextHandler(f.log, nil)),
+	})
 
 	f.route()
 	f.waitFor(routewatch.RequestFailed)