feat: Add better coverage for tx pool API tests

[tcoratger]

At the moment in #1390 we are currently only able to insert one transaction at a time into the mempool for mocking (due to limitations pointed out in #1383).

When this is resolved, it would be nice to improve the coverage in these tests by inserting for example 10 transactions into the mempool and checking that all of them are integrated into the endpoints related to the pool content transactions.

We should do something like

  // Create 10 samples transactions
  let transactions = create_sample_transactions(&katana_empty, 10)
      .await
      .expect("Failed to create sample transaction")
      .pop()
      .expect("Expected at least one transaction");

    for transaction in transactions {
      // Insert the transaction into the mempool
          let _tx_hash = katana_empty
              .eth_client
              .mempool()
              .add_transaction(TransactionOrigin::Local, transaction)
              .await
              .expect("Failed to insert transaction into the mempool");
      }