Fix formatting and add closing brace in tests.rs

Author: shruti2522

oscars/src/alloc/arena2/tests.rs

@@ -148,6 +148,8 @@ fn max_recycled_cap_respected() {
     assert_eq!(allocator.heap_size(), 0);
     // The recycled list holds exactly max_recycled pages.
     assert_eq!(allocator.recycled_arenas.len(), 4);
+}
+
 // === test for TaggedPtr::as_ptr === //
 
 // `TaggedPtr::as_ptr` must use `addr & !MASK` to unconditionally clear the high