ocaml-cfdp: use test.ml runner convention for the eio test sublib
Match the pattern used by the rest of the monorepo: keep [test_cfdp_eio.ml]
as the suite definition and add a [test.ml] entry point that calls
[Alcotest.run]. The dune stanza becomes [(test (name test) ...)] in line with
the parent [test/dune].