test: speedup p2p_feefilter.py by whitelisting peers (immediate tx relay)

Most of the test time is spent in wait_for_invs() after sending to addresses,
i.e. the bottleneck is in relaying transactions. By whitelisting the peers via
-whitelist, the inventory is transmissioned immediately rather than on average
every 5 seconds, speeding up the test significantly:

before:
$ time ./p2p_feefilter.py
...
real    0m39.367s
user    0m1.227s
sys 0m0.571s

with this commit:
$ time ./p2p_feefilter.py
...
real    0m9.386s
user    0m1.120s
sys 0m0.577s

test/functional/p2p_feefilter.py

@@ -54,7 +54,12 @@ class FeeFilterTest(BitcoinTestFramework):
         # mempool and wallet feerate calculation based on GetFee
         # rounding down 3 places, leading to stranded transactions.
         # See issue #16499
-        self.extra_args = [["-minrelaytxfee=0.00000100", "-mintxfee=0.00000100"]] * self.num_nodes
+        # grant noban permission to all peers to speed up tx relay / mempool sync
+        self.extra_args = [[
+            "-minrelaytxfee=0.00000100",
+            "-mintxfee=0.00000100",
+            "-whitelist=noban@127.0.0.1",
+        ]] * self.num_nodes
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()