[vector api] RFR: Fine-tune test cases for reduction operations

[Yang Zhang (Arm Technology China)]

> So, the root problem is that channeling a value through the heap limits the tests we can perform on it. Right?
Yes, I think it is.

> Another approach would be to always operate on sub-word results as ints/longs. But I'm fine with your proposal. Feel free to push it.
Thanks very much for you review. I have pushed it.

Regards
Yang