refactor(test): remove compare_byte_array util and rework tests

This is legacy from when we didn't have PartialEq and had limited constructors.
Note 1: `PartialEq` also compares the pending word and it's index, it's just no longer necessary to
explicitly mention it, and probably better not to given that it's an implementation detail.

Note 2: `format!` can also be used to create the expected here, but omitted to reduce the different
APIs each unit test touches.

Author: Gilad Chase