Use informative names for examples used in tests

[alyssadai]

• consider re-using a few informatively named example .json + .tsv's
• consider using more "human understandable" syntax for referring to subjects and session in example data (see Adding assessment tool attribute #62 (comment))

Now that we have decided to implement a required --dataset-dir argument and to make the .tsv and .json file paths optional (with the file names being participants.tsv participants.json by default), it may be cleaner to:

• store each of the example file pairs in its own informatively-named subdirectory inside tests/data

so that we can use the same, default file names within each of those subdirectories and simplify the current parameterization of tests + test CLI runner invocations.

• also, consider moving bids-examples and neurobagel_examples into data/ ?

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