Restore InnoDB Buffer Pool after tests

Tests should not affect the buffer pool, which is based on a real workload. So we should:
* save its state in init_status() via innodb_buffer_pool_dump_now (http://dev.mysql.com/doc/refman/5.6/en/innodb-parameters.html#sysvar_innodb_buffer_pool_dump_now)
* restore old state in deinit_status using innodb_buffer_pool_load_now

Note that those variables only exists in My5.6+ and Maria10.0+