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

test/cases/switch.1.zig

@@ -1,11 +1,13 @@
 pub fn main() u8 {
     var val: u8 = 2;
+    _ = &val;
     var a: u8 = switch (val) {
         0, 1 => 2,
         2 => 3,
         3 => 4,
         else => 5,
     };
+    _ = &a;
 
     return a - 3;
 }