test: Added push_back tests

All working

src/arraylist.c

@@ -142,7 +142,7 @@ ArrayListErr arraylist_push_back(ArrayList *arr, void *data) {
         }
     }
 
-    memcpy(arr->buffer, data, arr->elem_size);
+    memcpy(arr->buffer + (arr->len * arr->elem_size), data, arr->elem_size);
     arr->len++;
     return ARRLIST_OK;
 }