Testcase for wallet issue with orphaned rewards.

This adds a new test case demonstrating the wallet issue when block
rewards are orphaned (https://github.com/bitcoin/bitcoin/issues/14148).

test/functional/test_runner.py

@@ -283,6 +283,7 @@ BASE_SCRIPTS = [
     'feature_logging.py',
     'feature_anchors.py',
     'feature_coinstatsindex.py',
+    'wallet_orphanedreward.py',
     'p2p_node_network_limited.py',
     'p2p_permissions.py',
     'feature_blocksdir.py',