Add test file for hidden methods

Author: jamillambert

integration_test/tests/hidden.rs

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: CC0-1.0
+
+//! Tests for methods that are `== Hidden ==` and not in the API docs of Bitcoin Core.
+
+#![allow(non_snake_case)] // Test names intentionally use double underscore.
+
+use integration_test::{Node, NodeExt as _};
+use node::vtype::*; // All the version specific types.
+use node::mtype;