Showcase behavior of the `dune describe contexts` subcommand

  $ cat >dune-project <<EOF
  > (lang dune 3.14)
  > EOF

  $ cat > dune-workspace << EOF
  > (lang dune 3.14)
  > 
  > (context default)
  > 
  > (context
  >  (default
  >   (name alt)))
  > EOF

  $ dune describe contexts
  alt
  default
