test: consolidate mempool constants #36611

nit: replace further usage of the literal:

<details> <summary>git diff on 4385d415eb</summary>

diff --git a/test/functional/mempool_package_rbf.py b/test/functional/mempool_package_rbf.py
index c8f6a7c11a..6a2a011476 100755
--- a/test/functional/mempool_package_rbf.py
+++ b/test/functional/mempool_package_rbf.py
@@ -196,7 +196,7 @@ class PackageRBFTest(BitcoinTestFramework):
         parent_coins = self.coins[:num_coins]
         del self.coins[:num_coins]
 
-        # Original transactions: 101 transactions with 2 descendants each -> 303 total transactions, 101 clusters
+        # Original transactions: num_coins transactions with 2 descendants each -> num_coins * 3 total transactions, num_coins clusters
         size_three_clusters = []
         for coin in parent_coins:
             size_three_clusters.append(self.wallet.send_self_transfer_chain(from_node=node, chain_length=3, utxo_to_spend=coin))
@@ -211,13 +211,13 @@ class PackageRBFTest(BitcoinTestFramework):
         parent_fee_per_conflict = 10000
         child_feerate = 10000 * DEFAULT_FEE
 
-        # Conflict against all transactions by double-spending each parent, causing 101 cluster conflicts
+        # Conflict against all transactions by double-spending each parent, causing num_coins cluster conflicts
         package_parent = self.wallet.create_self_transfer_multi(utxos_to_spend=parent_coins, fee_per_output=parent_fee_per_conflict)
         package_child = self.wallet.create_self_transfer(fee_rate=child_feerate, utxo_to_spend=package_parent["new_utxos"][0])
 
         pkg_results = node.submitpackage([package_parent["hex"], package_child["hex"]], maxfeerate=0)
         assert_equal("transaction failed", pkg_results["package_msg"])
-        assert_equal(f"too many potential replacements, rejecting replacement {package_parent['txid']}; too many conflicting clusters (101 > 100)", pkg_results["tx-results"][package_parent["wtxid"]]["error"])
+        assert_equal(f"too many potential replacements, rejecting replacement {package_parent['txid']}; too many conflicting clusters ({num_coins} > {MAX_REPLACEMENT_CANDIDATES})", pkg_results["tx-results"][package_parent["wtxid"]]["error"])
         self.assert_mempool_contents(expected=expected_txns)
 
         # Make singleton tx to conflict with in next batch
@@ -233,7 +233,7 @@ class PackageRBFTest(BitcoinTestFramework):
         package_child = self.wallet.create_self_transfer(fee_rate=child_feerate, utxo_to_spend=package_parent["new_utxos"][0])
         pkg_results = node.submitpackage([package_parent["hex"], package_child["hex"]], maxfeerate=0)
         assert_equal("transaction failed", pkg_results["package_msg"])
-        assert_equal(f"too many potential replacements, rejecting replacement {package_parent['txid']}; too many conflicting clusters (101 > 100)", pkg_results["tx-results"][package_parent["wtxid"]]["error"])
+        assert_equal(f"too many potential replacements, rejecting replacement {package_parent['txid']}; too many conflicting clusters ({num_coins} > {MAX_REPLACEMENT_CANDIDATES})", pkg_results["tx-results"][package_parent["wtxid"]]["error"])
         self.assert_mempool_contents(expected=expected_txns)
 
         # Finally, conflict with MAX_REPLACEMENT_CANDIDATES clusters

</details>

nit: docstring could be preserved:

<details> <summary>git diff on 4385d415eb</summary>

diff --git a/test/functional/feature_taproot.py b/test/functional/feature_taproot.py
index 7a1a2d9a6b..2cba934648 100755
--- a/test/functional/feature_taproot.py
+++ b/test/functional/feature_taproot.py
@@ -1638,6 +1638,7 @@ class TaprootTest(BitcoinTestFramework):
                     fail_input is None  # Must be valid to be standard
                     and (all(utxo.spender.is_standard for utxo in input_utxos))  # All inputs must be standard
                     and tx.version in TX_STANDARD_VERSIONS # The tx version must be standard
+                    # test doesn't cover in-mempool spends, so only TRUC_MAX_VSIZE limit is hit
                     and not (tx.version == 3 and tx.get_vsize() > TRUC_MAX_VSIZE)  # Topological standardness rules must be followed
                 )
                 msg = ','.join(utxo.spender.comment + ("*" if n == fail_input else "") for n, utxo in enumerate(input_utxos))

</details>

nit: Could move comment to remaining definition of DEFAULT_CLUSTER_LIMIT in mempool_util.py.

Moved.