Add tests for send transaction sync

Currently there are no tests for [send raw txn sync](https://github.com/base/node-reth/blob/main/crates/flashblocks-rpc/src/rpc.rs#L236-L277) , we should add some tests for this to the integration [setup](https://github.com/base/node-reth/blob/main/crates/flashblocks-rpc/src/tests/rpc.rs)