Disable auth codes for Observatory test (#8646)

shell/testing/observatory/launcher.dart

@@ -42,6 +42,7 @@ class ShellLauncher {
     '--observatory-port=0',
     '--non-interactive',
     '--run-forever',
+    '--disable-service-auth-codes',
   ];
   final String shellExecutablePath;
   final String mainDartPath;