Test the (dialect ...) stanza inside the `dune-project` file.

  $ dune exec ./main.exe

  $ dune build @fmt 2>&1 | grep -v "fake ocamlformat is running"
  Formatting main.mfi
  File "fmt.ml", line 1, characters 0-0:
  --- fmt.ml
  +++ .formatted/fmt.ml
  @@ -1 +1 @@
  -prerr_endline ("Formatting " ^ Sys.argv.(1))
  +(* fake ocamlformat output *)
  \ No newline at end of file
  File "main.ml", line 1, characters 0-0:
  --- main.ml
  +++ .formatted/main.ml
  @@ -1 +1 @@
  -let () = ()
  +(* fake ocamlformat output *)
  \ No newline at end of file
  [1]

  $ dune trace cat | jq -r '
  > include "dune";
  >   processes
  > | select(.args.prog | basename == "ocamlformat")
  > | .args.stderr
  > | gsub("^\\s+|\\s+$"; "")
  > ' | sort
  fake ocamlformat is running: "--impl" "fmt.ml"
  fake ocamlformat is running: "--impl" "main.ml"
