[network] Mark flat-overlay jumbo tests as special_infra to avoid failures in unsupported clusters (#4092)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Updated test infrastructure: added a new pytest marker to tag a public
test class for special infra handling, ensuring selected connectivity
tests are marked for targeted runs.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: yossisegev

tests/network/flat_overlay/test_flat_overlay.py

@@ -93,6 +93,7 @@ def test_flat_overlay_consistent_ip(
 
 
 @pytest.mark.jumbo_frame
+@pytest.mark.special_infra
 class TestFlatOverlayJumboConnectivity:
     @pytest.mark.polarion("CNV-10162")
     @pytest.mark.s390x