Currently the per-tx sigops limit standardness check (bounded by
`MAX_STANDARD_TX_SIGOPS_COST`, throwing "bad-txns-too-many-sigops"
if exceeded) is only indirectly tested with the much higher per-block
consensus limit (`MAX_BLOCK_SIGOPS_COST`), i.e. an increase in the
limit would still pass all tests. Refine that by splitting up the invalid
tx template `TooManySigops` in a per-block and a per-tx one.
The involved functional tests taking use of these templates are
`feature_block.py` and `p2p_invalid_txs.py`.