[red-knot] Add a build.rs file to red_knot_python_semantic, and document pitfalls of using rstest in combination with mdtest (#13747)

crates/red_knot_python_semantic/build.rs

@@ -0,0 +1,4 @@
+/// Rebuild the crate if a test file is added or removed from
+pub fn main() {
+    println!("cargo:rerun-if-changed=resources/mdtest");
+}