CakeFixtureManager::load now calls CakeTestFixture::truncate

The `CakeFixtureManager::loadSingle` method calls `$this->_setupTable`, followed by `$fixture->truncate` before inserting fixture data into the db.

On the other hand, the `CakeFixtureManager::load` method does not call `$fixture->truncate` after setting up the table.  This behavior can cause problems (e.g. InnoDB Foreign key constraint errors) if `CakeTestCase::$dropTables` is set to false.

This change simply updates `CakeFixtureManager::load` to conform with the same behavior as `CakeFixtureManager::loadSingle`