Fix tx-pool returning the same transaction multiple times (#6535)

* Fix tx-pool returning the same transaction multiple times

This fixes a bug that lead to returning the same transaction multiple
times when iterating the `ready` iterator. Internally the transaction
was kept in the `best` list and could be duplicated in that list be
re-inserting it again. This `best` list is using a `TransactionRef`
which internally uses a `insertion_id`. This `insertion_id` could lead
to the same transaction being inserted multiple times into the `best`
list.

* Update client/transaction-pool/src/testing/pool.rs

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>

Co-authored-by: Nikolay Volf <nikvolf@gmail.com>