Merge bitcoin/bitcoin#26417: test: fix intermittent failure in feature_index_prune.py

201b9a0 test: fix intermittent failure in feature_index_prune.py (Martin Zumsande)

Pull request description:

  I can't reproduce the error from #26630 locally, but from analying the logs I think the problem is the following:

  After calling `sync_blocks`, we didn't check that the indexes have caught up to the tip before performing the manual pruning. This could possibly lead to prune blockers with a lower height than the expected 2489, which do appear in the logs of the failed CI runs, e.g.
   - `2022-10-27T21:14:17.703920Z [C:\Users\ContainerAdministrator\AppData\Local\Temp\cirrus-ci-build\src\validation.cpp:2395] [FlushStateToDisk] [prune] coinstatsindex limited pruning to height 2488` ([Cirrus](https://cirrus-ci.com/task/5443742333665280?logs=functional_tests#L2506))

  So, this should be fixed by a call to `sync_index`.
  Fixes #26330

ACKs for top commit:
  brunoerg:
    crACK 201b9a0

Tree-SHA512: fb7023c9eb2ba6d0e69e059a401453cbdf63abc6804543dffcf36ba9f93c9cd13209e57aa5536d94b2e420c9d4cd0b1a7eff1adadd19aa7b3c33f592502e1bc0

Author: MacroFake

test/functional/feature_index_prune.py

@@ -138,6 +138,7 @@ def run_test(self):
             self.connect_nodes(i, 3)
 
         self.sync_blocks(timeout=300)
+        self.sync_index(height=2500)
 
         for node in self.nodes[:2]:
             with node.assert_debug_log(['limited pruning to height 2489']):