plotextractor: fix remove_dups() test cases
plotextractor: fix remove_dups() test cases

* Fix test cases for remove_dups() as per addition of the context parameter.

* Prettify unit test output by adding descriptive test case docstrings.

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