Bug 28734: Clear cache to prevent random failures

Bug 28734: Clear cache to prevent random failures

MSS is modified, we must clear the cache before and after

    #   Failed test 'Authorised value is correctly parsed to show description rather than code'
    #   at t/db_dependent/Koha/Biblio.t line 617.
    #          got: 'CODE'
    #     expected: 'Description should show'

    #   Failed test 'Authorised value is correctly parsed for OPAC to show description rather than code'
    #   at t/db_dependent/Koha/Biblio.t line 624.
    #          got: 'CODE'
    #     expected: 'Description should show OPAC'
    # Looks like you failed 2 tests of 13

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>