core/txpool/legacypool: fix flaky test TestAllowedTxSize #30975

[stevemilk]

Some tests involving transactions near the txMaxSize limit were flaky.
This was due to ECDSA signatures occasionally having leading zeros, which are omitted during RLP encoding — making the final transaction size 1 byte smaller than expected.

To address this, a new helper function pricedDataTransactionWithFixedSignature was added.
It ensures both r and s are exactly 32 bytes (i.e., no leading zeros), producing transactions with deterministic size.

Test result:

9m40s: 715 runs so far, 0 failures
9m45s: 725 runs so far, 0 failures
9m50s: 725 runs so far, 0 failures

[jwasinger]

pricedDataTransaction already has a parameter to specify the size. Seems like that function should be fixed instead of creating a wrapper around it?