Transaction - Add test on Transaction

[stombre]

Currently, the test scenario on Transaction include the three ways to do transaction with ilorm with findOne and save operation.

Some scenarios cases for Transaction are missing, this issue could eventually be done by different PR and different test;

• Test instance.remove operator on transaction
• Test throw an error in a Transaction.run (to trigger rollback).
• Test rollback by calling the transaction.rollback operator

Any other cases that seems missing could be added.