Test melange.emit promotion

  $ cat > dune-project <<EOF
  > (lang dune 3.8)
  > (using melange 0.1)
  > EOF

  $ cat > dune <<EOF
  > (melange.emit
  >  (alias dist)
  >  (modules hello)
  >  (emit_stdlib false)
  >  (promote (until-clean))
  >  (target dist))
  > EOF

  $ cat > hello.ml <<EOF
  > let () =
  >   print_endline "hello"
  > EOF

  $ dune build @dist

Targets are promoted to the source tree

  $ ls ./dist
  hello.js

  $ node ./dist/hello.js
  hello

(until-clean) causes targets to be deleted after calling dune clean

  $ dune clean
  $ ls ./dist