Add explicit test for dataset without session level

[Remi-Gau]

Mostly as a sanity check as I have seen a lot of bids apps struggling to handle both of those cases.

Should be possible to use nilearn internals to mock fmriprep datasets as the test dataset only cover one of those.

[htwangtw]

Right now the test dataset has session but internally we have run several datasets without the session level at this point and the problem is solved. We can add an explicit test for this later.

[Remi-Gau]

okidoki!!

[htwangtw]

This will be addressed by adding ds000228 and it will address more issues beyond the session label. I will close this one in favour of #151!