test: Added tests made for state consistency

Also changed a few things like usin asser_null and assert_non_null
--didn't knew they existed in cmocka, or at least forgot-- for better
clarity and readability.

src/arraylist.c

@@ -89,7 +89,7 @@ bool arraylist_is_empty(const ArrayList *arr) {
     if (arr == NULL) {
         return true;
     }
-    return arr->capacity == 0;
+    return arr->len == 0;
 }
 
 static ArrayListErr arraylist_grow(ArrayList *arr) {