Prioritize `module.builder_kwargs` over defaults in `TestCommand`
{'login': 'lvwerra', 'id': 8264887, 'node_id': 'MDQ6VXNlcjgyNjQ4ODc=', 'avatar_url': 'https://avatars.githubusercontent.com/u/8264887?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/lvwerra', 'html_url': 'https://github.com/lvwerra', 'followers_url': 'https://api.github.com/users/lvwerra/followers', 'following_url': 'https://api.github.com/users/lvwerra/following{/other_user}', 'gists_url': 'https://api.github.com/users/lvwerra/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/lvwerra/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/lvwerra/subscriptions', 'organizations_url': 'https://api.github.com/users/lvwerra/orgs', 'repos_url': 'https://api.github.com/users/lvwerra/repos', 'events_url': 'https://api.github.com/users/lvwerra/events{/privacy}', 'received_events_url': 'https://api.github.com/users/lvwerra/received_events', 'type': 'User', 'site_admin': False}
[]
[]
This fixes a bug in the `TestCommand` where multiple kwargs for `name` were passed if it was set in both default and `module.builder_kwargs`. Example error:

```Python
Traceback (most recent call last):
  File "create_metadata.py", line 96, in <module>
    main(**vars(args))
  File "create_metadata.py", line 86, in main
    metadata_command.run()
  File "/opt/conda/lib/python3.7/site-packages/datasets/commands/test.py", line 144, in run
    for j, builder in enumerate(get_builders()):
  File "/opt/conda/lib/python3.7/site-packages/datasets/commands/test.py", line 141, in get_builders
    name=name, cache_dir=self._cache_dir, data_dir=self._data_dir, **module.builder_kwargs
TypeError: type object got multiple values for keyword argument 'name'
```

Let me know what you think.
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