Add e2e tests for build/diff kustomization

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

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

@@ -0,0 +1,12 @@
+apiVersion: v1
+data:
+  password: cGFzc3dvcmQK
+  username: YWRtaW4=
+kind: Secret
+metadata:
+  labels:
+      kustomize.toolkit.fluxcd.io/name: podinfo
+      kustomize.toolkit.fluxcd.io/namespace: {{ .fluxns }}
+  name: db-user-pass-bkbd782d2c
+  namespace: default
+type: Opaque