[qa] Test nMinimumChainWork

Nodes don't consider themselves out of "initial block download" until
their active chain has more work than nMinimumChainWork.

While in initial block download, nodes won't relay blocks to their
peers, so test that this parameter functions as intended by verifying
that block relay only succeeds past a given node once its
nMinimumChainWork has been exceeded.

test/functional/test_runner.py

@@ -121,6 +121,7 @@ BASE_SCRIPTS= [
     'bip65-cltv-p2p.py',
     'uptime.py',
     'resendwallettransactions.py',
+    'minchainwork.py',
 ]
 
 EXTENDED_SCRIPTS = [