Add e2e tests for build/diff kustomization

Signed-off-by: Soule BA <soule@weave.works>

cmd/flux/testdata/diff-kustomization/service.yaml

@@ -0,0 +1,21 @@
+apiVersion: v1
+kind: Service
+metadata:
+  labels:
+      kustomize.toolkit.fluxcd.io/name: podinfo
+      kustomize.toolkit.fluxcd.io/namespace: {{ .fluxns }}
+  name: podinfo
+  namespace: default
+spec:
+  type: ClusterIP
+  selector:
+    app: podinfo
+  ports:
+    - name: http
+      port: 9899
+      protocol: TCP
+      targetPort: http
+    - port: 9999
+      targetPort: grpc
+      protocol: TCP
+      name: grpc