944f7cc3f0
Adds golden-file tests for the new --signing-key-secret and --signing-key-type flags: no-signing (baseline), default-gpg (asserts type: gpg is rendered explicitly when only the secret is set), ssh, and the two validation-error cases. Establishes cmd/flux/testdata/create_image_update/ for future expansion of this command's coverage. Signed-off-by: Hidde Beydals <hidde@hhh.computer>