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

test/cases/structs.2.zig

@@ -2,6 +2,7 @@ const Example = struct { x: u8, y: u8 };
 
 pub fn main() u8 {
     var example: Example = .{ .x = 5, .y = 10 };
+    _ = &example;
     return example.y + example.x - 15;
 }