Minor fixes to get proper detection of transaction isolation level when
inside external_lock().


mysql-test/t/disabled.def:
  Enabling innodb.test again.
mysql-test/t/innodb.test:
  Fixing some error codes. Test currently requires MIXED or ROW mode since
  READ-COMMITTED and READ-UNCOMMITTED are not safe for STATEMENT mode, i.e.,
  when the binary log is enabled.
storage/innobase/handler/ha_innodb.cc:
  Using tx_isolation since trx->isolation_level is not defined when
  inside external_lock().