Add e2e tests for build/diff kustomization

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

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

@@ -0,0 +1,11 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+- ./deployment.yaml
+- ./hpa.yaml
+- ./service.yaml
+secretGenerator:
+ - literals:
+   - username=admin
+   - password=1f2d1e2e67df
+   name: secret-basic-auth