Add topics to pubspec.yaml

Topics help users on pub.dev discover packages and related packages.

You can see a list of [topics here](https://pub.dev/topics).
You can also make your own topics.

Topics are added to pubspec.yaml as follows:

```topics:
 - my-topic
 - some-other-topic
```

See also: https://dart.dev/tools/pub/pubspec#topics

pubspec.yaml

@@ -5,6 +5,10 @@ description: >-
   behavior verification, and stubbing.
 repository: https://github.com/dart-lang/mockito
 
+topics:
+ - testing
+ - mocking
+
 environment:
   sdk: ^3.3.0