BibIndex: new regression tests

• Adds tests for remove_stopwords, remove_html_markup, remove_latex_markup.

• Refactors other tests in bibindexadmin_regression_tests.py.

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