DocExtract: fix reference extractor unit tests
DocExtract: fix reference extractor unit tests

* Reference extractor's unit tests were not following Invenio convention,
  so running `inveniocfg --run-unit-tests` were not catching them.  This
  commit fixes the problem.

inveniosoftware/invenio,tiborsimko/invenio,inveniosoftware/invenio,tiborsimko/invenio