BibIndex: clean warning messages in test suite

* Mutes warnings caused by SQL DROP TABLE query.  (closes #1615)

* Fix for improper use of search_unit_in_bibrec function
  inside WordTable class.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>