The previous value for parent was not a valid commit ID. In a regular
compose request, the parent ref is resolved before being added as a
source in the manifest. However, when building test manifests, since
the values aren't real, we don't resolve the ostree refs, meaning that
the test manifests weren't valid — they failed validity checks if run in
osbuild.
Replaced the fake parent ref with a fake parent commit ID
"ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff".
The manifests still aren't buildable, since they don't point to a valid
ostree repository or commit, but now they're at least valid (in the
technical sense).
