ext: CrossRef Flask extension tests

* Adds tests for DOI found and not found.

* NOTE: for the search to be performed in the database, the DOI field
  prefix for the corresponding instance config file should be added to
  invenio.cfg as DOI_SEARCH_PREFIX (e.g. DOI_SEARCH_PREFIX = u'doi:').

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>