Fix a Python 3 warning

```
/home/simon/projects/servo/python/servo/testing_commands.py:301:
SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if err is not 0:
```

python/servo/testing_commands.py

@@ -298,7 +298,7 @@ def test_unit(self, test_name=None, package=None, bench=False, nocapture=False,
                 args += ["--", "--nocapture"]
 
             err = self.run_cargo_build_like_command("bench" if bench else "test", args, env=env, **kwargs)
-            if err is not 0:
+            if err:
                 return err
 
     @Command('test-content',