Add test for syncing blocks generated after invalidateblock.

Suhas wrote this test to demonstrate a bug in invalidateblock back
in 2015, and at some point between then and now it started passing,
so best to just merge it so we don't regress.

Closes #5806 (the original issue in which this test was written)

test/functional/p2p_post_invalidate_sync.py

@@ -0,0 +1,44 @@
+#!/usr/bin/env python3
+# Copyright (c) 2014 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#
+# Test invalidateblock
+#
+
+from test_framework.test_framework import BitcoinTestFramework
+
+class InvalidateBlockTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 2
+        self.setup_clean_chain = True
+        self.extra_args = [['-debug', '-whitelist=127.0.0.1'],['-debug', '-whitelist=127.0.0.1']]
+
+    def run_test(self):
+        self.nodes[0].generate(1) # Leave IBD
+        self.sync_all()
+
+        cnt = self.nodes[0].getblockcount()
+
+        node1blocks = self.nodes[1].generate(18)
+
+        self.sync_all()
+        if (self.nodes[0].getblockcount() != cnt + 18):
+            raise AssertionError("Failed to sync initial blocks")
+
+        self.nodes[0].invalidateblock(node1blocks[0])
+        self.nodes[1].invalidateblock(node1blocks[0])
+
+        if (self.nodes[0].getblockcount() != cnt):
+            raise AssertionError("Failed to invalidate initial blocks")
+
+        self.nodes[1].generate(17)
+
+        print("All blocks generated, trying to sync")
+        self.sync_all()
+        if (self.nodes[0].getblockcount() != cnt + 17):
+            raise AssertionError("Failed to sync shorter but valid chain")
+
+if __name__ == '__main__':
+    InvalidateBlockTest().main()

test/functional/test_runner.py

@@ -169,6 +169,7 @@
     'rpc_preciousblock.py',
     'wallet_importprunedfunds.py',
     'p2p_leak_tx.py',
+    'p2p_post_invalidate_sync.py',
     'rpc_signmessage.py',
     'wallet_balance.py',
     'feature_nulldummy.py',