test: add randomness e2e coverage

[Lchangliang]

Summary

• add single-node e2e coverage for the randomness-by-height precompile and Solidity wrapper usage
• switch the single-node alpha hardfork config to alphaTime for the timestamp-gated reth change
• build the randomness Solidity test contracts inside the Docker e2e runner so the new cases have fresh artifacts

Verification

• RUSTFLAGS='--cfg tokio_unstable' cargo build --bin gravity_node --profile quick-release
• python3 gravity_e2e/runner.py --force-init --no-cleanup single_node -k 'test_gravity_randomness_solidity_wrapper or test_gravity_prevrandao_solidity_interfaces or test_randomness_by_height_precompile'
• bash -n gravity_e2e/run_docker.sh
• git diff --check origin/main..HEAD