Prioritize module.builder_kwargs over defaults in TestCommand

[lvwerra]

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:

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.

[lhoestq]

Thanks for the fix ! Feel free to merge if it's good for you