[tests] Add wallet_balance.py

Adds a test specifically to test the wallet's getbalance and
getunconfirmedbalance RPCs.

test/functional/test_runner.py

@@ -153,6 +153,7 @@ BASE_SCRIPTS = [
     'wallet_importprunedfunds.py',
     'p2p_leak_tx.py',
     'rpc_signmessage.py',
+    'wallet_balance.py',
     'feature_nulldummy.py',
     'mempool_accept.py',
     'wallet_import_rescan.py',