exercises(linked-list): test: fix for Zig 0.12.0-dev (#366)

ee7 says:
The latest Zig master release produced a compile error for this test:

    $ zig version
    0.12.0-dev.2063+804cee3b9
    $ bin/run-tests linked-list
    Running tests for linked-list...
    test_linked_list.zig:71:9: error: local variable is never mutated
        var list = List{};
            ^~~~
    test_linked_list.zig:71:9: note: consider using 'const'

Officially, the Zig track currently supports only Zig 0.11.0 (released
on 2023-08-04), but here it's easy to fix for Zig master while still
supporting Zig 0.11.0. At the time of writing, with this commit, every
exercise test file on the track works with the latest Zig master
release.

exercises/practice/linked-list/test_linked_list.zig

@@ -68,7 +68,7 @@ test "pop, push, shift, and unshift can be used in any order" {
 }
 
 test "count an empty list" {
-    var list = List{};
+    const list = List{};
     try testing.expectEqual(@as(usize, 0), list.len);
 }