[google_maps_flutter][iOS 12] Skip testTakeSnapshot (flutter#6120)

Skips `testTakeSnapshot` test re-enabeld by flutter/packages#5823, but causing failures and blocking the `flutter/packages` tree, e.g. https://ci.chromium.org/ui/p/flutter/builders/prod/Mac_arm64%20ios_platform_tests_shard_2%20master/1697/overview.

packages/google_maps_flutter/google_maps_flutter_ios/example/ios12/integration_test/google_maps_test.dart

@@ -849,7 +849,9 @@ void main() {
         await controllerCompleter.future;
     final Uint8List? bytes = await controller.takeSnapshot();
     expect(bytes?.isNotEmpty, true);
-  });
+  },
+      // TODO(stuartmorgan): Re-enable; see https://github.com/flutter/flutter/issues/139825
+      skip: true);
 
   testWidgets(
     'set tileOverlay correctly',