dev: move `modexp` generated test data as e2e tests [1]

inside ssj `modexp.cairo` we added test data generated from a python file.
Instead, we could make this an e2e test.

Depends on: https://github.com/kkrt-labs/kakarot/issues/1596
Also depends on: enabling `modexp` as `external_precompile` in the CairoZero codebase