[PERF] tests data uses 8k inodes

[bpinsard]

I noticed that https://github.com/bids-standard/pybids/tree/master/bids/tests/data/ds005 does account for 8k files for each install in venvs, or for each Ci job that install pybids, which could slow things down depending on filesystem.

I just wanted to check, is that really necessary? Is that to stress/load-test pybids? Could it be outsourced and fetched rather than stored in this repo?

@effigies