Fix file type checks in data splits for contrastive training example script

[npyoung]

What does this PR do?

After checking if the validation file is a json or csv, existing code checks again to make sure it is only a json. This PR checks if the test file is a json or csv, as with validation and training, and does not force the validation file to be json.

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• Did you write any new necessary tests?

[amyeroberts]

Thanks!