* core: allow price bump at threshold
* core: test changes to allow price bump at threshold
* core: reinstate tx replacement test underneath threshold
* core: minor test failure message cleanups
As per #14661 TransactionByHash always returns false for pending.
This uses blockNumber rather than blockHash to ensure that it returns
the correct value for pending and will not suffer side-effects if
eth_getTransactionByHash is fixed in future.