BUG: train_test_dur_split_inds can return indices for training set slightly smaller than target duration

[NickleDave]

I am finding when generating multiple splits for BioSoundSegBench that sometimes vak.prep.split.split.train_test_dur_split_inds returns indices for a training set that is just slightly smaller than the target duration, e.g. 599.5 seconds instead of 600s.

Not sure how this happens -- we should have a post-condition to guard against it -- but generating multiple splits with a fixed dataset seems like a good start for replicating it