Tests: do not link with lwt.ssl if it is not installed
Tests: do not link with lwt.ssl if it is not installed

cdtest.ml uses lwt.ssl, and the Makefile won't build it if lwt.ssl is
not available.
cdtest_tls works fine with just tls.lwt, but if conduit got built with
lwt.ssl, then lwt.ssl is needed during linking, otherwise it is not.

Fixes https://github.com/ocaml/opam-repository/issues/8166

djs55/ocaml-conduit,rgrinberg/ocaml-conduit,samoht/ocaml-conduit,samoht/ocaml-conduit,rgrinberg/ocaml-conduit,mirage/ocaml-conduit,avsm/ocaml-conduit,djs55/ocaml-conduit