ocaml-tls: remove deleted tests/ directory and reorganize fuzz test
- Remove old tests/ directory (replaced by test/ with proper structure)
- Move fuzz.ml to eio/tests/fuzz/ subdirectory with its own dune file
- Update eio/tests/dune to remove fuzz stanza (now in subdirectory)
- Add test_helpers dependency to fuzz test (provides Mock_rng, Mock_socket)
- Fix merlint e617 pattern matching for newer OCaml AST (Pconst_string)