test: fix flaky wallet_send functional test

Rather than asserting that the exact fees are the same, check the fee rate rounded to nearest interger. This will account for small differences in fees caused by variability in ECDSA signature lengths.

Author: m3dwards

test/functional/wallet_send.py

@@ -563,6 +563,7 @@ def run_test(self):
             options={"inputs": [ext_utxo], "input_weights": [{"txid": ext_utxo["txid"], "vout": ext_utxo["vout"], "weight": 1000}]}
         )
 
+        target_fee_rate_sat_vb = 10
         # Funding should also work when input weights are provided
         res = self.test_send(
             from_wallet=ext_wallet,
@@ -572,14 +573,17 @@ def run_test(self):
             add_inputs=True,
             psbt=True,
             include_watching=True,
-            fee_rate=10
+            fee_rate=target_fee_rate_sat_vb
         )
         signed = ext_wallet.walletprocesspsbt(res["psbt"])
         signed = ext_fund.walletprocesspsbt(res["psbt"])
         assert signed["complete"]
         testres = self.nodes[0].testmempoolaccept([signed["hex"]])[0]
         assert_equal(testres["allowed"], True)
-        assert_fee_amount(testres["fees"]["base"], testres["vsize"], Decimal(0.0001))
+        actual_fee_rate_sat_vb = Decimal(testres["fees"]["base"]) * Decimal(1e8) / Decimal(testres["vsize"])
+        # Due to ECDSA signatures not always being the same length, the actual fee rate may be slightly different
+        # but rounded to nearest integer, it should be the same as the target fee rate
+        assert_equal(round(actual_fee_rate_sat_vb), target_fee_rate_sat_vb)
 
 if __name__ == '__main__':
     WalletSendTest().main()