t8n should use the transaction rlp provided by the testing framwork

> Transactions are the only part of the t8n which use some sort of a json round trip workaround in the PR. In order to avoid this workaround (and fully make use of the rlp that the testing framework generates) we will need to make some small changes on the specs side, particularly wrt the `decode_transaction` function. I aim to tackle this in a subsequent PR.

_Originally posted by @gurukamath in [#2924](https://github.com/ethereum/execution-specs/pull/2924/changes#r3309968102)_