Ticket 48364 - Fix test failures
    
    Description: New failures appear in the test suites. They should be
    fixed as soon as possible. More new features are coming to lib389(like
    REST or krb5 support) and they require a proper CI testing.
    
    Fix description: Remove "+" sign from multiline string in the
    lib389/backend.py, because it makes an impossible to replace string with
    variables on the second line.
    
    - tests/backend_test.py
    Fix exception naming in the tests, because it was updated with recent
    patches within lib389/backend.py module.
    
    - tests/suffix_test.py
    Refactor code to the pytest compatibility.
    
    https://fedorahosted.org/389/ticket/48364
    
    Reviewed by: wibrown (Thanks!)