Add tests for RequestPublishedArticleForm

Write test for bug from last commit.

Add tests to VetCommentary view, however
 the fixtures for groups remain empty (no permissions)
 causing problems with the ContributorFactory. Need some
 fix first, to have the groups work properly with the
 factories.

Add test to check expected ValueError
on processing form before validation.

Add more VetCommentary tests for the
three different possible valid form actions.

Add valid test for accepted `VetCommentaryForm`.
Further added factory for unvetted Commentary.

Method test_form_with_duplicate_*'s second part
 gave a false-positive result as the form_data
 contained neither a DOI nor a Arxiv ID. The
 tests came back positive but actually they were
 not.

Add invalid form test for RequestCommentaryForm.
Tests contain having no Arxiv ID or DOI or trying to
 give an already existing identifier.

Add first form tests using Factory to test RequestCommentaryForm.