Drop test tables only if exists, to prevent errors on clean database

See #261 & #272 for reference.

On clean database the tables foo and bar do not exists during the first run, so we get errors like:

    1) Zend_Test_PHPUnit_Db_Integration_MysqlIntegrationTest::testZendDbTableDataSet
    Zend_Db_Statement_Exception: SQLSTATE[42S02]: Base table or view not found: 1051 Unknown table 'foo', query was: DROP TABLE foo

This PR adds IF EXISTS clause to the drop query, making it work even on new and empty database.