bridge descs: Add tests for process_document
@nickm, please confirm the accuracy of the following comment which I add in this new test case:
// TODO ideally we would test the `ops::Bound::Unbounded` case in process_download's
// expiry time handling, but that would require making a document with unbounded
// validity time. Even if that is possible, I don't think we have code in-tree to
// make signed test documents.