add test to demonstrate pair of "half" kernels sharing same public excess

[antiochp]

• Added new build_two_half_kernels test
• Cleanup test transaction building code (so we can pass explicit excess in)

The test code used to simply generate a random offset but we can now force it to use the appropriate offset to compensate for a specific excess value during tx building and kernel signing.

We intend to do something similar to this for NRD kernels once we have support for multi-party outputs. In real-world usage both participants would provide the necessary offset to adjust the final public excess back to a value shared across two transactions.

[quentinlesceller]

Few comments. Still some files to review.

[quentinlesceller]

LGTM 👍