Add unit tests for export

Signed-off-by: Somtochi Onyekwere <somtochionyekwere@gmail.com>

cmd/flux/testdata/export/ks.yaml

@@ -0,0 +1,14 @@
+---
+apiVersion: kustomize.toolkit.fluxcd.io/v1beta2
+kind: Kustomization
+metadata:
+  name: flux-system
+  namespace: {{ .fluxns }}
+spec:
+  interval: 5m0s
+  path: ./infrastructure/
+  prune: true
+  sourceRef:
+    kind: GitRepository
+    name: flux-system
+