test: add 'flux diff ks' tests for cases that involve new namespaces

Signed-off-by: rycli <cyril@ryc.li>
Assisted-by: claude-code/claude-opus-4-6

cmd/flux/testdata/build-kustomization/new-namespace-only/kustomization.yaml

@@ -0,0 +1,4 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+- ./namespace.yaml

cmd/flux/testdata/build-kustomization/new-namespace-only/namespace.yaml

@@ -0,0 +1,4 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: new-ns