Add unit tests for export

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

cmd/flux/testdata/export/image-update.yaml

@@ -0,0 +1,20 @@
+---
+apiVersion: image.toolkit.fluxcd.io/v1beta1
+kind: ImageUpdateAutomation
+metadata:
+  name: flux-system
+  namespace: {{ .fluxns }}
+spec:
+  git:
+    commit:
+      author:
+        email: fluxcdbot@users.noreply.github.com
+        name: fluxcdbot
+  interval: 1m0s
+  sourceRef:
+    kind: GitRepository
+    name: flux-system
+  update:
+    path: ./clusters/my-cluster
+    strategy: Setters
+