[test] CScriptNum Decode Check as Unit Tests

Migrates the CScriptNum decode tests into a unit test, and moved some
changes made in #14816. Made possible by the integration of
test_framework unit testing in #18576. Further extends the original
test with larger ints, similar to the scriptnum_tests.cpp file. Adds
test to blocktools.py testing fn create_coinbase() with CScriptNum
decode.

test/functional/test_framework/blocktools.py

@@ -4,6 +4,8 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Utilities for manipulating blocks and transactions."""
 
+import unittest
+
 from .address import (
     key_to_p2sh_p2wpkh,
     key_to_p2wpkh,
@@ -217,3 +219,9 @@ def send_to_witness(use_p2wsh, node, utxo, pubkey, encode_p2sh, amount, sign=Tru
             tx_to_witness = ToHex(tx)
 
     return node.sendrawtransaction(tx_to_witness)
+
+class TestFrameworkBlockTools(unittest.TestCase):
+    def test_create_coinbase(self):
+        height = 20
+        coinbase_tx = create_coinbase(height=height)
+        assert_equal(CScriptNum.decode(coinbase_tx.vin[0].scriptSig), height)

test/functional/test_framework/script.py

@@ -731,3 +731,9 @@ class TestFrameworkScript(unittest.TestCase):
         self.assertEqual(bn2vch(0xFFFFFFFF), bytes([0xFF, 0xFF, 0xFF, 0xFF, 0x00]))
         self.assertEqual(bn2vch(123456789), bytes([0x15, 0xCD, 0x5B, 0x07]))
         self.assertEqual(bn2vch(-54321), bytes([0x31, 0xD4, 0x80]))
+
+    def test_cscriptnum_encoding(self):
+        # round-trip negative and multi-byte CScriptNums
+        values = [0, 1, -1, -2, 127, 128, -255, 256, (1 << 15) - 1, -(1 << 16), (1 << 24) - 1, (1 << 31), 1 - (1 << 32), 1 << 40, 1500, -1500]
+        for value in values:
+            self.assertEqual(CScriptNum.decode(CScriptNum.encode(CScriptNum(value))), value)