More test for the new BIDS strutucture

[bruAristimunha]

trafficstars

          We need more tests

Originally posted by @bruAristimunha in https://github.com/NeuroTechX/moabb/pull/408#discussion_r1279901661

[PierreGtch]

Yes agreed, I only tested the BIDS conversion for a few datasets.

I don't think these new tests should be executed every time we push a new commit to GitHub (too heavy) but only once in a while when we decide.