fixtures: dynamic introduction of [unit] within [dataset_semantics] is not allowed

[tiborsimko]

Current behaviour

When loading TOTEM records from cernopendata/opendata.cern.ch#3693 into a local CERN Open Data portal instance running with v0.1.11 image (and the portal source code being checked out at 2943d1d, the dataset record loading leads to the following trouble with dataset semantics information:

$ docker exec -t opendatacernch-web-1 cernopendata fixtures records --mode insert-or-replace -f /content/data/records/totem-datasets-2011.json
...
opensearchpy.exceptions.RequestError: RequestError(400, 'strict_dynamic_mapping_exception', 'mapping set to strict, dynamic introduction of [unit] within [dataset_semantics] is not allowed')
...
  File "/opt/invenio/var/instance/python/lib/python3.9/site-packages/opensearchpy/connection/base.py", line 315, in _raise_error
    raise HTTP_EXCEPTIONS.get(status_code, TransportError)(

Loading of the same data file without the dataset_semantics.unit subfield present works well.

Expected behaviour

If we support dataset_semantics.unit subfield as a free text value, and the record fixtures supply some values, then the record loading should be working fine.