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.
dateutils: fix for unit test suite

* Use absolute imports so that `inveniocfg --run-unit-tests` would
  run this test suite.
WebSearch: empty unit test suite for summarizer

* Fixes otherwise empty summarizer unit test suite so that
  `inveniocfg --run-unit-tests` would not complain about
  missing TEST_SUITE.
Merge branch 'master' into next

* master:
  dateutils: fix for unit test suite
  DocExtract: fix reference extractor unit tests
Merge branch 'master' into next

* master:
  WebSearch: empty unit test suite for summarizer