test: fix intermittent failure in feature_index_prune.py

After syncing the blocks, we didn't check that the
indexes have caught up to the tip before manually pruning.
This could lead to prune blockers lower thatn the expected height.

test/functional/feature_index_prune.py

@@ -138,6 +138,7 @@ class FeatureIndexPruneTest(BitcoinTestFramework):
             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']):