The Mtime field is mapped as an OpenSearch `date`, which rejects an
empty value with `mapper_parsing_exception: cannot parse empty date`.
The folder and root fixtures had no Mtime, so serializing them to
`"Mtime": ""` made TestEngine_Purge/purge_resource_trees fail when the
document was indexed. Give both a valid RFC3339 Mtime, matching the
file fixture.
Dominik Schmidt
7cc144a9c3