Merge bitcoin#32924: test: add valid tx test with minimum-sized ECDSA signature (8 bytes DER-encoded)

5fa81e2 test: add valid tx test with minimum-sized ECDSA signature (8 bytes DER-encoded) (Sebastian Falbesoner)

Pull request description:

  Currently in our tests, all ECDSA signatures passing verification have sizes of 69 bytes and above (that's the DER-encoded size, i.e. counted without the sighash flag byte) [1]. This PR adds test coverage for the minimum-sized valid case of 8 bytes, by taking an interesting testnet transaction that I stumbled upon:
  https://mempool.space/testnet/tx/c6c232a36395fa338da458b86ff1327395a9afc28c5d2daa4273e410089fd433
  Note that this is a very obscure construction that only works because the public key used isn't contained in the locking script, but calculated and provided later at spending time (see https://bitcointalk.org/index.php?topic=1729534.msg17309060#msg17309060 for an explainer), to match the message (sighash) and picked signature. So this doesn't represent a use-case that really makes sense in practice, but it can still appear in a block (not in mempool though, due to `SCRIPT_VERIFY_CONST_SCRIPTCODE`), and having test-coverage seems useful.

  Can be tested with same patch below (tests crash with the condition `>= 9`, but pass with `>= 8`).

  [1] this can be verified by applying the following patch and running the tests:
  ```diff
  diff --git a/src/pubkey.cpp b/src/pubkey.cpp
  index a4ca9a1..bee0caa603 100644
  --- a/src/pubkey.cpp
  +++ b/src/pubkey.cpp
  @@ -288,7 +288,9 @@ bool CPubKey::Verify(const uint256 &hash, const std::vector<unsigned char>& vchS
       /* libsecp256k1's ECDSA verification requires lower-S signatures, which have
        * not historically been enforced in Bitcoin, so normalize them first. */
       secp256k1_ecdsa_signature_normalize(secp256k1_context_static, &sig, &sig);
  -    return secp256k1_ecdsa_verify(secp256k1_context_static, &sig, hash.begin(), &pubkey);
  +    bool ret = secp256k1_ecdsa_verify(secp256k1_context_static, &sig, hash.begin(), &pubkey);
  +    if (ret) assert(vchSig.size() >= 69);
  +    return ret;
   }
  ```

ACKs for top commit:
  ajtowns:
    ACK 5fa81e2 lgtm
  fjahr:
    tACK 5fa81e2
  real-or-random:
    utACK bitcoin@5fa81e2 interesting case

Tree-SHA512: d1f0612fdb71c9238ca0420f574f6f246e60dbd11970b23f21d082c759a89ff98a13b12a1f6266f14f20539ec437b7ab79322082278da32984ddfee2d8893356

Author: fanquake

src/test/data/tx_valid.json

@@ -31,6 +31,10 @@
 [[["60a20bd93aa49ab4b28d514ec10b06e1829ce6818ec06cd3aabd013ebcdc4bb1", 0, "1 0x41 0x04cc71eb30d653c0c3163990c47b976f3fb3f37cccdcbedb169a1dfef58bbfbfaff7d8a473e7e2e6d317b87bafe8bde97e3cf8f065dec022b51d11fcdd0d348ac4 0x41 0x0461cbdcc5409fb4b4d42b51d33381354d80e550078cb532a34bfa2fcfdeb7d76519aecc62770f5b0e4ef8551946d8a540911abe3e7854a26f39f58b25c15342af 2 OP_CHECKMULTISIG"]],
 "0100000001b14bdcbc3e01bdaad36cc08e81e69c82e1060bc14e518db2b49aa43ad90ba26000000000494f47304402203f16c6f40162ab686621ef3000b04e75418a0c0cb2d8aebeac894ae360ac1e780220ddc15ecdfc3507ac48e1681a33eb60996631bf6bf5bc0a0682c4db743ce7ca2b01ffffffff0140420f00000000001976a914660d4ef3a743e3e696ad990364e555c271ad504b88ac00000000", "DERSIG,LOW_S,STRICTENC,NULLDUMMY"],
 
+["The following is c6c232a36395fa338da458b86ff1327395a9afc28c5d2daa4273e410089fd433 on testnet3"],
+["It contains an OP_CHECKSIG with the shortest valid DER encoded signature (8 bytes w/o sighash flag), i.e. 3006020101020101 (r=1, s=1)"], [[["cf016927962ec028964c186043d48e465b3d4672f758953b00d3c4682f71cad6", 0, "HASH160 0x14 0x58a994e9d5ed9baa03ecfd1137592a90ad3cdfc5 EQUAL"]],
+"0100000001d6ca712f68c4d3003b9558f772463d5b468ed44360184c9628c02e96276901cf000000002f21026d2204a9535443657a88a0724fbd49a0e78d305f50a82f2cc9dd9bea10a6c5cd0c093006020101020101017cacffffffff010000000000000000016a00000000", "CONST_SCRIPTCODE"],
+
 ["The following is c99c49da4c38af669dea436d3e73780dfdb6c1ecf9958baa52960e8baee30e73"],
 ["It is of interest because it contains a 0-sequence as well as a signature of SIGHASH type 0 (which is not a real type)"],
 [[["406b2b06bcd34d3c8733e6b79f7a394c8a431fbf4ff5ac705c93f4076bb77602", 0, "DUP HASH160 0x14 0xdc44b1164188067c3a32d4780f5996fa14a4f2d9 EQUALVERIFY CHECKSIG"]],