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

test/cases/switch.3.zig

@@ -2,7 +2,8 @@ const MyEnum = enum { One, Two, Three };
 
 pub fn main() u8 {
     var val: MyEnum = .Two;
-    var a: u8 = switch (val) {
+    _ = &val;
+    const a: u8 = switch (val) {
         .One => 1,
         .Two => 2,
         .Three => 3,