_beh.tsv files are not subject to the same stimuli file existence checks as events.tsv.

[rwblair]

events.tsv has their own validation section here:
https://github.com/bids-standard/bids-validator/blob/master/bids-validator/validators/tsv/tsv.js#L126

beh.tsv files should skip the onset and duration validation but should be subject to any other validation that events.tsv files are.

[effigies]

This needs a test case.