Tests: Test new getblockstats RPC

Includes commit from Anthony Towns @ajtowns:

Tests: Save and load block and corresponding expected statistics

test/functional/test_runner.py

@@ -135,6 +135,7 @@ BASE_SCRIPTS = [
     'wallet_resendwallettransactions.py',
     'wallet_fallbackfee.py',
     'feature_minchainwork.py',
+    'rpc_getblockstats.py',
     'p2p_fingerprint.py',
     'feature_uacomment.py',
     'p2p_unrequested_blocks.py',