adapt bumpfee change discard test to be more strict and add note on p2sh discrep

test/functional/wallet_bumpfee.py

@@ -181,7 +181,10 @@ def test_dust_to_fee(rbf_node, dest_address):
     # the bumped tx sets fee=49,900, but it converts to 50,000
     rbfid = spend_one_input(rbf_node, dest_address)
     fulltx = rbf_node.getrawtransaction(rbfid, 1)
-    bumped_tx = rbf_node.bumpfee(rbfid, {"totalFee": 49900})
+    # (32-byte p2sh-pwpkh output size + 148 p2pkh spend estimate) * 10k(discard_rate) / 1000 = 1800
+    # P2SH outputs are slightly "over-discarding" due to the IsDust calculation assuming it will
+    # be spent as a P2PKH.
+    bumped_tx = rbf_node.bumpfee(rbfid, {"totalFee": 50000-1800})
     full_bumped_tx = rbf_node.getrawtransaction(bumped_tx["txid"], 1)
     assert_equal(bumped_tx["fee"], Decimal("0.00050000"))
     assert_equal(len(fulltx["vout"]), 2)