Test: Add tests for functions in helpers.cairo

[Jagadeeshftw]

Description

This PR adds tests for the helper functions in helpers.cairo. The new tests cover:

• Conversion logic for board_to_pos and pos_to_board, including normal moves and home stretch conditions, with special handling for threshold values and not-placed pieces.
• Various scenarios for pos_reducer with different active color combinations and orders (e.g., [0, 1], [0, 3], [2, 1]), including cases where some values are already non-zero and should be preserved.

Related Issue(s)

Closes #206

Checklist:

• Read the contributing docs (if this is your first contribution)

• Verified this is not a duplicate of any existing pull request

• Performed a self-review of the code

• Rebased to the last commit of the target branch (or merged it into my branch)

• Linked the issues which this PR resolves

• Documented the changes in code (API docs will be generated automatically)

[co-lead]

Thanks for your PR @Jagadeeshftw. It will be reviewed shortly.