I'm doing the same test knit as J-J. I decided to not only because I love the pattern but the general idea seems to be to follow the pattern and use the tools and methods you want. It starts with a provisional cast of choice and no requirement of a specific one. I agreed to follow the pattern when I took on the test knitting, totally my decision and choice, so ithere are no designer control freaking issues to contend with. I knew I'd be following the pattern. I soon realized I can do the cables thus far without a cable needle so again, my choice. After a huge amount of frustration because sometimes I just miss something it's been enjoyable so far. I'm not sure I can meet the deadline but I'll do my utmost to finish on time.And that would work and still enable you to do the pattern as it is written.
But it seems to me that if you are doing a test knit you need to be prepared to have your knitting controlled. You can't just change something in the pattern if you want to and so you are being controlled. You can't adjust to in the round or flat because you prefer it that way so you are being controlled.
So unless like using markers to mark needles rather than using DPNs (which is what I would do as well as it is effectively using DPNs as you can still have needles numbered and I don't like DPNs either) you need to follow the pattern exactly as written. And cable length would fit this category, using a smaller length as long as can still follow the pattern exactly it won't matter. And indeed it would be one of the points you would make to the designer that the cable length seems to be far too long.
Of course if you are following a pattern rather than doing a test knit then how closely you stick to the pattern is totally up to you.