Merge bitcoin/bitcoin#32463: test: fix an incorrect feature_fee_estimation.py subtest

9b75cfd test: retain the intended behavior of `feature_fee_estimation.py` nodes (ismaelsadeeq)
5c1236f test: fix incorrect subtest in `feature_fee_estimation.py` (ismaelsadeeq)

Pull request description:

  Attempt to fix #32461

  In the `estimatesmartfee` RPC, we return the maximum of the following: the feerate estimate for the target, `minrelaytxfee`, and `mempoolminfee`.

  https://github.com/bitcoin/bitcoin/blob/9a05b45da60d214cb1e5a50c3d2293b1defc9bb0/src/rpc/fees.cpp#L85

  The test `test_feerate_mempoolminfee`, originally introduced in bitcoin/bitcoin@ea31caf, is incorrect.

  It should calculate the fee rate ceiling by taking the maximum of the custom `minrelaytxfee`, `mempoolminfee`, and the highest fee rate observed during the test (`check_smart_estimates`). This is necessary because:

  * There is no guarantee that the generated fee rates will exceed both `minrelaytxfee` and `mempoolminfee`.
  * Users can start a node with custom fee settings.

  Due to the non-deterministic nature of the `feature_fee_estimation.py` test, it often passes by chance. The randomly generated fees typically include a value higher than the custom `minrelaytxfee`, inadvertently hiding the issue.

  Issue #32461 identified a random seeds that consistently fails the test because the generated fees never exceed the custom `minrelaytxfee`:

  e.g
  ```
  build/test/functional/feature_fee_estimation.py --random=3450808900320758527
  ```

  This PR has two commits which :

  * Correctly fixes the test by calculating the fee rate ceiling as the maximum of the node `minrelaytxfee`, `mempoolminfee`, and the highest seen fee rate, when verifying smart fee estimates.
  * Improves the subtest name and comment for clarity.
  * Restores the original test behavior by appending 4000 WU to the custom `blockmaxweight`.

ACKs for top commit:
  achow101:
    ACK 9b75cfd
  glozow:
    ACK 9b75cfd
  theStack:
    Light ACK 9b75cfd

Tree-SHA512: 0f7fb0496b50a399b58f6fb1afd95414fad454795fbc0046e22dfc54a2062ae0c519a12ebfeb6ad7ef547438868d99eca2351c0d19d0346adaadb500eff6f15f

Author: achow101

test/functional/feature_fee_estimation.py

@@ -90,19 +90,20 @@ def check_smart_estimates(node, fees_seen):
     """Call estimatesmartfee and verify that the estimates meet certain invariants."""
 
     delta = 1.0e-6  # account for rounding error
-    last_feerate = float(max(fees_seen))
     all_smart_estimates = [node.estimatesmartfee(i) for i in range(1, 26)]
     mempoolMinFee = node.getmempoolinfo()["mempoolminfee"]
     minRelaytxFee = node.getmempoolinfo()["minrelaytxfee"]
+    feerate_ceiling = max(max(fees_seen), float(mempoolMinFee), float(minRelaytxFee))
+    last_feerate = feerate_ceiling
     for i, e in enumerate(all_smart_estimates):  # estimate is for i+1
         feerate = float(e["feerate"])
         assert_greater_than(feerate, 0)
         assert_greater_than_or_equal(feerate, float(mempoolMinFee))
         assert_greater_than_or_equal(feerate, float(minRelaytxFee))
 
-        if feerate + delta < min(fees_seen) or feerate - delta > max(fees_seen):
+        if feerate + delta < min(fees_seen) or feerate - delta > feerate_ceiling:
             raise AssertionError(
-                f"Estimated fee ({feerate}) out of range ({min(fees_seen)},{max(fees_seen)})"
+                f"Estimated fee ({feerate}) out of range ({min(fees_seen)},{feerate_ceiling})"
             )
         if feerate - delta > last_feerate:
             raise AssertionError(
@@ -144,8 +145,8 @@ def set_test_params(self):
         self.noban_tx_relay = True
         self.extra_args = [
             [],
-            ["-blockmaxweight=68000"],
-            ["-blockmaxweight=32000"],
+            ["-blockmaxweight=72000"],
+            ["-blockmaxweight=36000"],
         ]
 
     def setup_network(self):
@@ -237,10 +238,10 @@ def sanity_check_estimates_range(self):
         self.log.info("Final estimates after emptying mempools")
         check_estimates(self.nodes[1], self.fees_per_kb)
 
-    def test_feerate_mempoolminfee(self):
-        high_val = 3 * self.nodes[1].estimatesmartfee(1)["feerate"]
+    def test_estimates_with_highminrelaytxfee(self):
+        high_val = 3 * self.nodes[1].estimatesmartfee(2)["feerate"]
         self.restart_node(1, extra_args=[f"-minrelaytxfee={high_val}"])
-        check_estimates(self.nodes[1], self.fees_per_kb)
+        check_smart_estimates(self.nodes[1], self.fees_per_kb)
         self.restart_node(1)
 
     def sanity_check_rbf_estimates(self, utxos):
@@ -451,11 +452,11 @@ def run_test(self):
         self.log.info("Test fee_estimates.dat is flushed periodically")
         self.test_estimate_dat_is_flushed_periodically()
 
-        # check that the effective feerate is greater than or equal to the mempoolminfee even for high mempoolminfee
+        # check that estimatesmartfee feerate is greater than or equal to maximum of mempoolminfee and minrelaytxfee
         self.log.info(
-            "Test fee rate estimation after restarting node with high MempoolMinFee"
+            "Test fee rate estimation after restarting node with high minrelaytxfee"
         )
-        self.test_feerate_mempoolminfee()
+        self.test_estimates_with_highminrelaytxfee()
 
         self.log.info("Test acceptstalefeeestimates option")
         self.test_acceptstalefeeestimates_option()