The kola update tests need a dev-key-signed update payload. This was
lacking and caused the update tests to be skipped.
Generate the test update payload for both dev builds and release builds
and run the kola tests for both. The test update payload has a special
name to not confuse it with the real update payload for releases, and
we keep the previous behavior to sign releases. Therefore, the
generate_update function wasn't used but the extract_update function
extended with generating the additional test payload.
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