ipn: don't listen on the unspecified address in test

To avoid the Mac firewall dialog of (test) death.

See 4521a59f30
which I added to help debug this.

ipn/e2e_test.go

@@ -13,6 +13,7 @@ import (
 	"net/http/cookiejar"
 	"net/http/httptest"
 	"net/url"
+	"os"
 	"strings"
 	"testing"
 	"time"
@@ -26,6 +27,13 @@ import (
 	"tailscale.io/control" // not yet released
 )
 
+func init() {
+	// Hacky way to signal to magicsock for now not to bind on the
+	// unspecified address. TODO(bradfitz): clean up wgengine's
+	// constructors.
+	os.Setenv("IN_TS_TEST", "1")
+}
+
 func TestIPN(t *testing.T) {
 	testy.FixLogs(t)
 	defer testy.UnfixLogs(t)