[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)