Add bignum test case generation script

[wernerlewis]

Description

Resolves #5921

Adds a test case generation framework for bignum tests, similar to generate_psa_tests.py. This PR adds a few bignum arithmetic test cases to show the usage of the test framework, and it will be expanded to cover more bignum/ECC functionality in subsequent PRs.

This PR should ideally be merged after #6070. The temporary radix argument commit can then be removed before merging this PR.

Status

READY

Requires Backporting

Yes 2.28