tstest/integration/vms: add in-process DERP server (#2108)

Previously this test would reach out to the public DERP servers in order
to help machines connect with eachother. This is not ideal given our
plans to run these tests completely disconnected from the internet. This
patch introduces an in-process DERP server running on its own randomly
assigned HTTP port.

Updates #1988

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

tstest/integration/vms/vms_test.go

@@ -546,7 +546,8 @@ func TestVMIntegrationEndToEnd(t *testing.T) {
 
 	rex := distroRex.Unwrap()
 
-	ln, err := net.Listen("tcp", deriveBindhost(t)+":0")
+	bindHost := deriveBindhost(t)
+	ln, err := net.Listen("tcp", net.JoinHostPort(bindHost, "0"))
 	if err != nil {
 		t.Fatalf("can't make TCP listener: %v", err)
 	}
@@ -555,6 +556,9 @@ func TestVMIntegrationEndToEnd(t *testing.T) {
 
 	cs := &testcontrol.Server{}
 
+	derpMap := integration.RunDERPAndSTUN(t, t.Logf, bindHost)
+	cs.DERPMap = derpMap
+
 	var (
 		ipMu  sync.Mutex
 		ipMap = map[string]ipMapping{}