Add e2e tests for build/diff kustomization

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

cmd/flux/testdata/build-kustomization/podinfo-source.yaml

@@ -0,0 +1,16 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: {{ .fluxns }}
+---
+apiVersion: source.toolkit.fluxcd.io/v1beta1
+kind: GitRepository
+metadata:
+  name: podinfo
+  namespace: {{ .fluxns }}
+spec:
+  interval: 30s
+  ref:
+    branch: master
+  url: https://github.com/stefanprodan/podinfo