First: This may be my lack of “classical” experience, but I’d never heard this particular scale referred to as “octatonic” before; I’ve always seen it called the “diminished” scale.
Second: I think the solution the tests accept for this exercise is incorrect. I assert that the C diminished scale should be written with flats and not sharps (as the test suggests). The interval from the tonic to the third is a minor third, and C minor (enharmonic to Eb major) is written with flats. This section of the Wikipedia article on the “Octatonic scale” backs this up. Looking at the diminished scale that contains C, you can see that the accidentals are all written with flats.
I think the accepted solution should be
["C", "D", "Eb", "F", "Gb", "Ab", "A", "B"]
. I’d be happy to submit a PR, but it looks like that’s currently not a thing that can happen.