test: apply fixed feerate to avoid variable dynamic fees

Without specifying a feerate, we let the wallet decide on an
appropriate feerate, which can be influenced by various factors
such as what's in the mempool. Since wallet_groups.py fails when
feerates are unstable, we should use a fixed feerate across all nodes.

Closes #25940

test/functional/wallet_groups.py

@@ -26,9 +26,10 @@ class WalletGroupTest(BitcoinTestFramework):
             ["-maxapsfee=0.00002719"],
             ["-maxapsfee=0.00002720"],
         ]
-        # whitelist peers to speed up tx relay / mempool sync
+
         for args in self.extra_args:
-            args.append("-whitelist=noban@127.0.0.1")
+            args.append("-whitelist=noban@127.0.0.1")   # whitelist peers to speed up tx relay / mempool sync
+            args.append(f"-paytxfee={20 * 1e3 / 1e8}")  # apply feerate of 20 sats/vB across all nodes
 
         self.rpc_timeout = 480