fix(lint): resolve E610/E615/E618 by extracting helpers into libraries
- ec_wycheproof.ml -> test/ec/ec_wycheproof/ library (fixes E610/E615/E618)
- random_selftests.ml -> tests/random_selftests/ library (fixes E610/E615/E618)
- check_vectors.ml -> test/check_vectors/ executable (fixes E618)
- Add random_selftests.mli (fixes E405)