Update testing dataset for new loader

To test the new loader, we want to add those modalities to the data-testing dataset:

- [x] microscopy PNG
- [ ] microscopy OMETIFF
- [ ] anat (2D single-slice nifti)
- [ ] dwi (4D nifti)
- [ ] eeg (files TBD)
- [x] sessions.tsv architectures and files
- [x] scans.tsv architectures and files

Start by inspecting the existing testing dataset entries for microscopy PNG, sessions.tsv, and scans.tsv, then compare them with the new loader's expected inputs. Add microscopy OMETIFF, anat 2D single-slice NIfTI, and dwi 4D NIfTI data; clarify the EEG file format before adding it. Done means all unchecked modalities are represented and the loader can consume them.

Written by the indexing model from the issue text.