std.meta: test that const and mut variables have different addresses

We are checking that two identical, constant values, are stored at
different addresses.

But sharing a unique location doesn't look like something the compiler
wouldn't do.

It may make more sense to check that a const variable and a mutable
variable set to the same value have different addresses.

lib/std/meta.zig

@@ -574,7 +574,7 @@ test "std.meta.eql" {
         .c = "54321".*,
     };
 
-    const s_3 = S{
+    var s_3 = S{
         .a = 134,
         .b = 123.3,
         .c = "12345".*,