Fixes the case where there is a PoolTxs of type Exit with fee 0 for a
TokenID. Before this commit the Coordinator was not sending the
accumulated fee (which has value 0) to the Coordinator account for that
TokenID, with this commit the Coordinator always sends the accumulated
fee even when the value is 0 to match the hermez circom circuits
behaviour.

Also added a test to check the values and that also sends the proof to a
real proof server to check that can generate a valid proof.

PoolL2Tx.Info contains information about the status & State of the
transaction. As for example, if the Tx has not been selected in the last
batch due not enough Balance at the Sender account, this reason would
appear at this parameter.
This will help the client (wallet, batchexplorer, etc) to reason why a
L2Tx is not selected in the forged batches.

- Add tests connecting TxSelector, BatchBuilder, ZKInputs, ProofServer
- Added test to check that the signatures of the PoolL2Txs from the L2DB
pool can be verified, to check that the parameters of the PoolL2Tx match
the original parameters signed before inserting them into the L2DB

- TxProcessor move txCompressedDataEmpty inside the if of tp.zki!=nil
- Abstract generation of transactions for ZKInput tests to avoid code
repetition
  - used at txprocessor & test/zkproof tests

Test TxProcessor zkInputs against a real proof server