test: update cases to silence 'var is never mutated' errors

test/cases/structs.3.zig

@@ -3,6 +3,7 @@ const Example = struct { x: u8, y: u8 };
 pub fn main() u8 {
     var example: Example = .{ .x = 5, .y = 10 };
     var example2: Example = .{ .x = 10, .y = 20 };
+    _ = &example2;
 
     example = example2;
     return example.y + example.x - 30;