test: add missing assert_equal imports

The later scripted diff only rewrites call sites.
Add `assert_equal` imports in the remaining files that still need them so the mechanical replacement can apply cleanly.

test/functional/test_framework/psbt.py

@@ -5,6 +5,7 @@
 
 import base64
 
+from .util import assert_equal
 from .messages import (
     CTransaction,
     deser_string,