Task transporter tests: Commit pending transaction in order to avoid

StorageTransactionError: Duplicate tpc_begin calls for same transaction.
See https://github.com/4teamwork/opengever.core/pull/556
Merge pull request #556 from 4teamwork/lg-avoid-duplicate_tpc_begin_calls_for_same_transaction

Task transporter tests: Commit pending transaction in order to avoid StorageTransactionError