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/sww_test.go

@@ -975,9 +975,20 @@ func TestStalledRemoteLogEndpointHoldsUpNoRequest(t *testing.T) {
 			t.Errorf("no line dropped in the metrics:\n%s", metrics)
 		}
 
-		// Closed, the endpoint refuses the connection made to send the
-		// lines still waiting at the stop, which then does not wait.
-		_ = endpoint.Close()
+		// From here the endpoint closes each connection it takes, so the
+		// one made to send the lines still waiting at the stop fails at
+		// once. Closing the endpoint instead would free its port for
+		// another test to take.
+		go func() {
+			for {
+				conn, err := endpoint.Accept()
+				if err != nil {
+					return
+				}
+
+				_ = conn.Close()
+			}
+		}()
 	})
 
 	out.line(t, "type", "request")