Send tip change notification from invalidateblock

This change is needed to prevent sync_blocks timeouts in the mempool_reorg
test after the sync_blocks update in the upcoming commit
"[qa] Change sync_blocks to pick smarter maxheight".

This change was initially suggested by Suhas Daftuar <sdaftuar@chaincode.com>
in https://github.com/bitcoin/bitcoin/pull/8680#r78209060

Github-Pull: #9196
Rebased-From: 67c6326abd

src/main.cpp

@@ -2999,6 +2999,7 @@ bool InvalidateBlock(CValidationState& state, const Consensus::Params& consensus
 
     InvalidChainFound(pindex);
     mempool.removeForReorg(pcoinsTip, chainActive.Tip()->nHeight + 1, STANDARD_LOCKTIME_VERIFY_FLAGS);
+    uiInterface.NotifyBlockTip(IsInitialBlockDownload(), pindex->pprev);
     return true;
 }