tests(attribute-in-enum): Add a test with a macro attribute for exhaustivity

Current naive implementation fails with multi-line macro attributes.

Author: malikolivier

tests/target/rust-doc-in-enum/horizontal-no-doc.rs renamed to tests/target/attribute-in-enum/horizontal-no-doc.rs

@@ -0,0 +1,13 @@
+enum A {
+    B {
+        a: usize,
+        b: usize,
+        c: usize,
+        d: usize,
+    },
+
+    #[attr]
+    C {
+        a: usize,
+    },
+}