Merge bitcoin/bitcoin#30893: test: Introduce ensure_for helper

111465d72d test: Remove unused attempts parameter from wait_until (Fabian Jahr)
5468a23eb9 test: Add check_interval parameter to wait_until (Fabian Jahr)
16c87d91fd test: Introduce ensure_for helper (Fabian Jahr)

Pull request description:

  A repeating pattern in the functional tests is that the test sleeps for a while to ensure that a certain condition is still true after some amount of time has elapsed. Most recently a new case of this was added in #30807. This PR here introduces an `ensure` helper to streamline this functionality.

  Some approach considerations:
  - It is possible to construct this by reusing `wait_until` and wrapping it in `try` internally. However, the logger output of the failing wait would still be printed which seems irritating. So I opted for simplified but similar internals to `wait_until`.
  - This implementation starts for a failure in the condition right away which has the nice side-effect that it might give feedback on a failure earlier than is currently the case. However, in some cases, it may be expected that the condition may still be false at the beginning and then turns true until time has run out, something that would work when the test sleeps without checking in a loop. I decided against this design (and even against adding it as an option) because such a test design seems like it would be racy either way.
  - I have also been going back and forth on naming. To me `ensure` works well but I am also not a native speaker, happy consider a different name if others don't think it's clear enough.

ACKs for top commit:
  maflcko:
    re-ACK 111465d72d 🍋
  achow101:
    ACK 111465d72d
  tdb3:
    code review re ACK 111465d72d
  furszy:
    utACK 111465d72d

Tree-SHA512: ce01a4f3531995375a6fbf01b27d51daa9d4c3d7cd10381be6e86ec5925d2965861000f7cb4796b8d40aabe3b64c4c27e2811270e4e3c9916689575b8ba4a2aa

test/functional/feature_assumeutxo.py

@@ -9,7 +9,6 @@ to a hash that has been compiled into bitcoind.
 The assumeutxo value generated and used here is committed to in
 `CRegTestParams::m_assumeutxo_data` in `src/kernel/chainparams.cpp`.
 """
-import time
 from shutil import rmtree
 
 from dataclasses import dataclass
@@ -31,6 +30,7 @@ from test_framework.util import (
     assert_approx,
     assert_equal,
     assert_raises_rpc_error,
+    ensure_for,
     sha256sum_file,
     try_rpc,
 )
@@ -305,8 +305,7 @@ class AssumeutxoTest(BitcoinTestFramework):
         # If it does request such blocks, the snapshot_node will ignore requests it cannot fulfill, causing the ibd_node
         # to stall. This stall could last for up to 10 min, ultimately resulting in an abrupt disconnection due to the
         # ibd_node's perceived unresponsiveness.
-        time.sleep(3)  # Sleep here because we can't detect when a node avoids requesting blocks from other peer.
-        assert_equal(len(ibd_node.getpeerinfo()[0]['inflight']), 0)
+        ensure_for(duration=3, f=lambda: len(ibd_node.getpeerinfo()[0]['inflight']) == 0)
 
         # Now disconnect nodes and finish background chain sync
         self.disconnect_nodes(ibd_node.index, snapshot_node.index)