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This allows to import more entities from siaf_shortened.xml, because entities
whose broader concept isn't in the file are now imported (the relation isn't
inlined anymore). Thus, we have to update unittest_sobjects as well (and cleanup
a bit the original RDF data, we don't need that much error sample, a single one
is enough).

Test for import of scheme without title in passing.

This makes it easier to debug tests.

As a result let SKOSParser inherit from DataFeedParser and just define its
`process` method (which accepts the `raise_on_error` argument).