tests: in-memory database instead of on-disk
tests: in-memory database instead of on-disk

* Uses SQLite in-memory database for tests instead of on-disk
  test.db.

Signed-off-by: Sami Hiltunen <ebafb54bedf33e7dd12950a4634d6a0f6ba63e44@cern.ch>

inveniosoftware/invenio-oauth2server,tiborsimko/invenio-oauth2server,inveniosoftware/invenio-oauth2server,inveniosoftware/invenio-oauth2server,tiborsimko/invenio-oauth2server,tiborsimko/invenio-oauth2server