Add more datamodule tests

[sfmig]

Especially for reproducible dataset splits (see #218 )

It would be nice to define a mini dataset in GIN we can fetch for testing purposes. ---> added in PR #242. See also #248

[sfmig]

• add tests for the generation of splits -- Fix validation and test split not being reproducible #218
• add tests for multiple image directories
• add test for _compute_list_of_transforms
• add test for setup()
• add tests for dataloaders