[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_runner.py

@@ -68,6 +68,7 @@ TEST_EXIT_SKIPPED = 77
 
 TEST_FRAMEWORK_MODULES = [
     "address",
+    "blocktools",
     "script",
 ]