Add a focus test for TXM

[ligurio]

TXM has been implemented in scope of Transaction engine for memtx engine #4897

It implements MVCC and bank would be a good test for it because bank is a canonical test for MVCC: "For instance, when making a wire transfer between two bank accounts if a reader reads the balance at the bank when the money has been withdrawn from the original account and before it was deposited in the destination account, it would seem that money has disappeared from the bank." via