Use a Counter in tests/link/test_vm

tests/link/test_vm.py

 import time
+from collections import Counter
 
 import numpy as np
 import pytest
@@ -34,11 +35,10 @@ class TestCallbacks:
     # Test the `VMLinker`'s callback argument, which can be useful for debugging.
 
     def setup_method(self):
-        self.n_callbacks = {}
+        self.n_callbacks = Counter()
 
     def callback(self, node, thunk, storage_map, compute_map):
         key = node.op.__class__.__name__
-        self.n_callbacks.setdefault(key, 0)
         self.n_callbacks[key] += 1
 
     def test_callback(self):
@@ -50,9 +50,9 @@ class TestCallbacks:
         )
 
         f(1, 2, 3)
-        assert sum(self.n_callbacks.values()) == len(f.maker.fgraph.toposort())
+        assert self.n_callbacks.total() == len(f.maker.fgraph.toposort())
         f(1, 2, 3)
-        assert sum(self.n_callbacks.values()) == len(f.maker.fgraph.toposort()) * 2
+        assert self.n_callbacks.total() == len(f.maker.fgraph.toposort()) * 2
 
     def test_callback_with_ifelse(self):
         a, b, c = scalars("abc")