refactor(crypto): move EC tests to test/, fix E610/E618 lint issues
Move EC test infrastructure from tests/ to test/ directory:
- test_crypto_ec.ml, test vectors, and wycheproof/test_common helpers
- Rename ec_wycheproof.ml -> test_ec_wycheproof.ml (E618)
- Rename random_selftests.ml -> test_random_selftests.ml (E618)
- Update tests/dune to drop EC-only dependencies