Tests for PSBT

Added functional tests for PSBT that test the RPCs. Also added all
of the BIP 174 test vectors (except for the updater tests) in the
functional tests.

Added a Unit test for the BIP 174 updater test vector.

test/functional/test_runner.py

@@ -99,6 +99,7 @@ BASE_SCRIPTS = [
     'wallet_multiwallet.py',
     'wallet_multiwallet.py --usecli',
     'interface_http.py',
+    'rpc_psbt.py',
     'rpc_users.py',
     'feature_proxy.py',
     'rpc_signrawtransaction.py',