test: refactor: move fill_mempool to new module mempool_util

This is needed to avoid circular dependencies in later commits.
Can be reviewed via `--color-moved=dimmed-zebra`.

test/functional/rpc_packages.py

@@ -8,6 +8,9 @@ from decimal import Decimal
 import random
 
 from test_framework.blocktools import COINBASE_MATURITY
+from test_framework.mempool_util import (
+    fill_mempool,
+)
 from test_framework.messages import (
     MAX_BIP125_RBF_SEQUENCE,
     tx_from_hex,
@@ -18,7 +21,6 @@ from test_framework.util import (
     assert_equal,
     assert_fee_amount,
     assert_raises_rpc_error,
-    fill_mempool,
 )
 from test_framework.wallet import (
     DEFAULT_FEE,