Prioritize `module.builder_kwargs` over defaults in `TestCommand`
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.