Relax MIN_STANDARD_TX_NONWITNESS_SIZE to 65 non-witness bytes

Since the original fix was set to be a "reasonable" transaction
to reduce allocations and the true motivation later revealed,
it makes sense to relax this check to something more principled.

There are more exotic transaction patterns that could take advantage
of a relaxed requirement, such as 1 input, 1 output OP_RETURN to burn
a utxo to fees for CPFP purposes when change isn't practical.

Two changes could be accomplished:

1) Anything not 64 bytes could be allowed

2) Anything above 64 bytes could be allowed

In the Great Consensus Cleanup, suggestion (2) was the route taken.
It would not allow an "empty" OP_RETURN
but would reduce the required padding from 22 bytes to 5.

The functional test is also modified to test the actual case
we care about: 64 bytes

test/functional/mempool_accept.py

@@ -14,7 +14,9 @@ from test_framework.messages import (
     MAX_BIP125_RBF_SEQUENCE,
     COIN,
     COutPoint,
+    CTransaction,
     CTxIn,
+    CTxInWitness,
     CTxOut,
     MAX_BLOCK_WEIGHT,
     MAX_MONEY,
@@ -26,13 +28,19 @@ from test_framework.script import (
     OP_0,
     OP_HASH160,
     OP_RETURN,
+    OP_TRUE,
 )
 from test_framework.script_util import (
+    DUMMY_MIN_OP_RETURN_SCRIPT,
     keys_to_multisig_script,
+    MIN_PADDING,
+    MIN_STANDARD_TX_NONWITNESS_SIZE,
     script_to_p2sh_script,
+    script_to_p2wsh_script,
 )
 from test_framework.util import (
     assert_equal,
+    assert_greater_than,
     assert_raises_rpc_error,
 )
 from test_framework.wallet import MiniWallet
@@ -333,6 +341,35 @@ class MempoolAcceptanceTest(BitcoinTestFramework):
             maxfeerate=0,
         )
 
+        # Prep for tiny-tx tests with wsh(OP_TRUE) output
+        seed_tx = self.wallet.send_to(from_node=node, scriptPubKey=script_to_p2wsh_script(CScript([OP_TRUE])), amount=COIN)
+        self.generate(node, 1)
+
+        self.log.info('A tiny transaction(in non-witness bytes) that is disallowed')
+        tx = CTransaction()
+        tx.vin.append(CTxIn(COutPoint(int(seed_tx[0], 16), seed_tx[1]), b"", SEQUENCE_FINAL))
+        tx.wit.vtxinwit = [CTxInWitness()]
+        tx.wit.vtxinwit[0].scriptWitness.stack = [CScript([OP_TRUE])]
+        tx.vout.append(CTxOut(0, CScript([OP_RETURN] + ([OP_0] * (MIN_PADDING - 2)))))
+        # Note it's only non-witness size that matters!
+        assert_equal(len(tx.serialize_without_witness()), 64)
+        assert_equal(MIN_STANDARD_TX_NONWITNESS_SIZE - 1, 64)
+        assert_greater_than(len(tx.serialize()), 64)
+
+        self.check_mempool_result(
+            result_expected=[{'txid': tx.rehash(), 'allowed': False, 'reject-reason': 'tx-size-small'}],
+            rawtxs=[tx.serialize().hex()],
+            maxfeerate=0,
+        )
+
+        self.log.info('Minimally-small transaction(in non-witness bytes) that is allowed')
+        tx.vout[0] = CTxOut(COIN - 1000, DUMMY_MIN_OP_RETURN_SCRIPT)
+        assert_equal(len(tx.serialize_without_witness()), MIN_STANDARD_TX_NONWITNESS_SIZE)
+        self.check_mempool_result(
+            result_expected=[{'txid': tx.rehash(), 'allowed': True, 'vsize': tx.get_vsize(), 'fees': { 'base': Decimal('0.00001000')}}],
+            rawtxs=[tx.serialize().hex()],
+            maxfeerate=0,
+        )
 
 if __name__ == '__main__':
     MempoolAcceptanceTest().main()