test: added coverage to rpc_scantxoutset.py

Included a test that checks if an invalid first argument is entered we
receive a rpc error. The rpc should fail if "start", "status" or "abort"
is not the first command.

Author: kevkevinpal

test/functional/rpc_scantxoutset.py

@@ -123,6 +123,9 @@ def run_test(self):
         # Check that second arg is needed for start
         assert_raises_rpc_error(-1, "scanobjects argument is required for the start action", self.nodes[0].scantxoutset, "start")
 
+        # Check that invalid command give error
+        assert_raises_rpc_error(-8, "Invalid action 'invalid_command'", self.nodes[0].scantxoutset, "invalid_command")
+
 
 if __name__ == "__main__":
     ScantxoutsetTest().main()