fix(k8s-functional): skip flush-and-reshard test for non-eks backends

The 'test_nodetool_flush_and_reshard' test is affected by the following
bug:

    scylladb/scylla-operator#1077

using non-fast K8S backends such as 'k8s-gke' and 'k8s-local-kind'.
So, skip it on the slow backends.

functional_tests/scylla_operator/test_functional.py

@@ -693,6 +693,9 @@ def test_default_dns_policy(db_cluster: ScyllaPodCluster):
         f"Pods: {yaml.safe_dump(pods_with_wrong_dns_policy, indent=2)}")
 
 
+# NOTE: non-fast K8S backends such as 'k8s-gke' and 'k8s-local-kind' are affected by following bug:
+#       https://github.com/scylladb/scylla-operator/issues/1077
+@pytest.mark.requires_backend("k8s-eks")
 def test_nodetool_flush_and_reshard(db_cluster: ScyllaPodCluster):
     target_node = db_cluster.nodes[0]