test: properly check for per-tx sigops limit

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`.

test/functional/test_framework/blocktools.py

@@ -49,6 +49,7 @@ from .util import assert_equal
 
 MAX_BLOCK_SIGOPS = 20000
 MAX_BLOCK_SIGOPS_WEIGHT = MAX_BLOCK_SIGOPS * WITNESS_SCALE_FACTOR
+MAX_STANDARD_TX_SIGOPS = 4000
 MAX_STANDARD_TX_WEIGHT = 400000
 
 # Genesis block time (regtest)