Disable flutter_portal tests (#479)

Disable flutter_portal tests. The flutter_portal CI runs on an old
Flutter SDK (3.10.0) and does not support patches on the current main
channel Flutter SDK.

From https://github.com/flutter/tests/blob/main/README.md:
* The upstream repository that hosts the tests must be able to receive
patches to support the main channel of Flutter. This means that CI on
the upstream repository should use the main channel Flutter SDK.

@fzyzcjy

Author: b-luk