Adds golden-file tests for the new --signing-key-secret and
--signing-key-type flags: no-signing (baseline), default-gpg (asserts
type is omitted when only the secret is set, deferring to the
controller's gpg default), 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>
Hidde Beydals