Merge bitcoin/bitcoin#27171: test: add coverage for sigop limit policy (-bytespersigop setting)

89cd20cbed test: add coverage for sigop limit policy (`-bytespersigop` setting) (Sebastian Falbesoner)

Pull request description:

  This PR adds missing test coverage for the `-bytespersigop` option, which determines how pre-taproot signature operations (OP_CHECKSIG{VERIFY}, OP_CHECKMULTIGSIG{VERIFY}) affect fee handling calculations. The setting was introduced in PR #7081 for mitigating the [sigop spam attack](https://bitcointalk.org/index.php?topic=1166928.0); the initial implementation rejected txs exceeding the limit, but was changed in #8365 later to account for higher sizes in the mempool (i.e. exceeding the sigop limit is possible, but has to be compensated by higher fees).

  For each combination of `-bytespersigop` setting and sigops count, the test first creates a P2WSH spending transaction with a witness script that puts sigops in a non-executing branch (OP_FALSE OP_IF OP_CHECKMULTISIG ... OP_CHECKSIG ... OP_ENDIF). This tx is then bumped up to reach exactly the _sig-op limit equivalent vsize_ by padding its datacarrier output. Based on that, increasing the tx's vsize should still reflect a vsize increase in the mempool, while a decrease of the tx's vsize should lead to the mempool treating the tx's vsize to be the _sig-op limit equivalent vsize_, since the limit was exceeded.

  I assume that this parameter is almost never set explicitly by users (also it is not relevant for taproot spends), but it doesn't hurt to have a test for it. See also https://bitcoin.stackexchange.com/a/87958 for another explanation.

ACKs for top commit:
  glozow:
    light review ACK 89cd20cbed
  MarcoFalke:
    nice ACK 89cd20cbed  📁

Tree-SHA512: 06998ce93bf9d5ce6143db2996a43f13990c415f97afe684227ad469349e73952bf4f6c871c1e6349e07606f4d45db64408848873a86a89481cdca5a134e5e60

test/functional/test_runner.py

@@ -323,6 +323,7 @@ BASE_SCRIPTS = [
     'mempool_compatibility.py',
     'mempool_accept_wtxid.py',
     'mempool_dust.py',
+    'mempool_sigoplimit.py',
     'rpc_deriveaddresses.py',
     'rpc_deriveaddresses.py --usecli',
     'p2p_ping.py',