Scene not marked as complete when used with files with time tolerance

[ninahakansson]

The problem with using time as key and finding files within a time tolerance was partly solved in
#124.

The files are now found but the scene is not marked as ready.
Add line assert slot.get_status() == Status.SLOT_READY to the test_mismatching_files_within_tolerance_generate_one_slot. To see where code fails.