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/mining_getblocktemplate_longpoll.py

@@ -8,7 +8,7 @@ import random
 import threading
 
 from test_framework.test_framework import BitcoinTestFramework
-from test_framework.util import get_rpc_proxy
+from test_framework.util import assert_equal, get_rpc_proxy
 from test_framework.wallet import MiniWallet