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/diff-kustomization/flux-kustomization-new-namespace-only.yaml

@@ -0,0 +1,13 @@
+---
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: new-namespace-only
+spec:
+  interval: 5m0s
+  path: ./kustomize
+  force: true
+  prune: true
+  sourceRef:
+    kind: GitRepository
+    name: new-namespace-only