fix: change servertailnet to register the DERP dialer before setting DERP map (#12137)

I noticed a possible race where tailnet.Conn can try to dial the embedded region before we've set our custom dialer that send the DERP in-memory.  This closes that race and adds a test case for servertailnet with no STUN and an embedded relay

tailnet/tailnettest/tailnettest.go

@@ -32,22 +32,47 @@ import (
 //go:generate mockgen -destination ./coordinatormock.go -package tailnettest github.com/coder/coder/v2/tailnet Coordinator
 //go:generate mockgen -destination ./coordinateemock.go -package tailnettest github.com/coder/coder/v2/tailnet Coordinatee
 
+type derpAndSTUNCfg struct {
+	DisableSTUN    bool
+	DERPIsEmbedded bool
+}
+
+type DERPAndStunOption func(cfg *derpAndSTUNCfg)
+
+func DisableSTUN(cfg *derpAndSTUNCfg) {
+	cfg.DisableSTUN = true
+}
+
+func DERPIsEmbedded(cfg *derpAndSTUNCfg) {
+	cfg.DERPIsEmbedded = true
+}
+
 // RunDERPAndSTUN creates a DERP mapping for tests.
-func RunDERPAndSTUN(t *testing.T) (*tailcfg.DERPMap, *derp.Server) {
+func RunDERPAndSTUN(t *testing.T, opts ...DERPAndStunOption) (*tailcfg.DERPMap, *derp.Server) {
+	cfg := new(derpAndSTUNCfg)
+	for _, o := range opts {
+		o(cfg)
+	}
 	logf := tailnet.Logger(slogtest.Make(t, nil))
 	d := derp.NewServer(key.NewNode(), logf)
 	server := httptest.NewUnstartedServer(derphttp.Handler(d))
 	server.Config.ErrorLog = tslogger.StdLogger(logf)
 	server.Config.TLSNextProto = make(map[string]func(*http.Server, *tls.Conn, http.Handler))
 	server.StartTLS()
-
-	stunAddr, stunCleanup := stuntest.ServeWithPacketListener(t, nettype.Std{})
 	t.Cleanup(func() {
 		server.CloseClientConnections()
 		server.Close()
 		d.Close()
-		stunCleanup()
 	})
+
+	stunPort := -1
+	if !cfg.DisableSTUN {
+		stunAddr, stunCleanup := stuntest.ServeWithPacketListener(t, nettype.Std{})
+		t.Cleanup(func() {
+			stunCleanup()
+		})
+		stunPort = stunAddr.Port
+	}
 	tcpAddr, ok := server.Listener.Addr().(*net.TCPAddr)
 	if !ok {
 		t.FailNow()
@@ -65,11 +90,12 @@ func RunDERPAndSTUN(t *testing.T) (*tailcfg.DERPMap, *derp.Server) {
 						RegionID:         1,
 						IPv4:             "127.0.0.1",
 						IPv6:             "none",
-						STUNPort:         stunAddr.Port,
+						STUNPort:         stunPort,
 						DERPPort:         tcpAddr.Port,
 						InsecureForTests: true,
 					},
 				},
+				EmbeddedRelay: cfg.DERPIsEmbedded,
 			},
 		},
 	}, d