Expected: '[5, 10, 16, 21]', instead got: '[0, 5, 10, 16, 21]'

0 is divisible by 5 and not excluded.

What's with the sample tests testing the function output with itself?

The only explanation I can think of is the author knows that there are only a few possible outputs and it's too easy to get the answer from the test results. If that's the case why are you publishing such an un-interesting kata in the first place?

Needs Random Tests.