tstest/integration/vms: attempt to send more UDP packets (#2433)

Apparently this test was flaking because I critically misunderstood how
the kernel buffers UDP packets for senders. I'm trying to send more UDP
packets and will see if that helps.

Signed-off-by: Christine Dodrill <xe@tailscale.com>

tstest/integration/vms/vms_test.go

@@ -602,9 +602,21 @@ func (h Harness) testDistro(t *testing.T, d Distro, ipm ipMapping) {
 		}
 		defer ln.Close()
 
+		ctx, cancel := context.WithCancel(context.Background())
+		t.Cleanup(cancel)
+
+		go func() {
+			for {
+				select {
+				case <-ctx.Done():
+					return
+				default:
+				}
+
 				sess, err := cli.NewSession()
 				if err != nil {
-			t.Fatalf("can't open session: %v", err)
+					t.Errorf("can't open session: %v", err)
+					return
 				}
 				defer sess.Close()
 
@@ -615,12 +627,15 @@ func (h Harness) testDistro(t *testing.T, d Distro, ipm ipMapping) {
 				_, port, _ := net.SplitHostPort(ln.LocalAddr().String())
 
 				cmd := fmt.Sprintf("/udp_tester -client %s\n", net.JoinHostPort("100.64.0.1", port))
-		time.Sleep(10 * time.Millisecond)
 				t.Logf("sending packet: %s", cmd)
 				err = sess.Run(cmd)
 				if err != nil {
-			t.Errorf("can't send UDP packet: %v", err)
+					t.Logf("can't send UDP packet: %v", err)
+				}
+
+				time.Sleep(10 * time.Millisecond)
 			}
+		}()
 
 		t.Log("listening for packet")
 		n, _, err := ln.ReadFromUDP(buf)