Cover signingKey round-trip in export tests

Extends the existing TestExport 'image update' case with a signingKey
block on the seeded ImageUpdateAutomation, asserting the new field
survives the kubeClient.Get + serialize path. Parallels how the
existing fixture exercises every other field on the resource.

Signed-off-by: Hidde Beydals <hidde@hhh.computer>

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

@@ -10,6 +10,10 @@ spec:
       author:
         email: fluxcdbot@users.noreply.github.com
         name: fluxcdbot
+      signingKey:
+        secretRef:
+          name: my-signing-key
+        type: ssh
   interval: 1m0s
   sourceRef:
     kind: GitRepository