test(melange): reproduce ocaml#7020 (ocaml#7025)

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

test/blackbox-tests/test-cases/melange/gh7020.t

@@ -0,0 +1,54 @@
+Reproduce github #7020
+
+  $ dir=_to-install
+  $ mkdir $dir
+  $ cat >$dir/dune-project <<EOF
+  > (lang dune 3.7)
+  > (using melange 0.1)
+  > (package
+  >  (name dummyfoo))
+  > EOF
+
+  $ cat >$dir/dune <<EOF
+  > (library
+  >  (modes melange)
+  >  (public_name dummyfoo))
+  > EOF
+
+  $ cat >$dir/META.dummyfoo.template <<EOF
+  > # DUNE_GEN
+  > EOF
+
+  $ dune build @install --root $dir
+  Entering directory '_to-install'
+  Leaving directory '_to-install'
+  $ cd $dir
+  $ dune install --root . --prefix _install
+  Installing _install/lib/dummyfoo/META
+  Installing _install/lib/dummyfoo/dummyfoo.ml
+  Installing _install/lib/dummyfoo/dune-package
+  Installing _install/lib/dummyfoo/melange/dummyfoo.cmi
+  Installing _install/lib/dummyfoo/melange/dummyfoo.cmj
+  Installing _install/lib/dummyfoo/melange/dummyfoo.cmt
+  $ cd ..
+
+  $ export OCAMLPATH=$PWD/$dir/_install/lib
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.7)
+  > (using melange 0.1)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (melange.emit
+  >  (target es6)
+  >  (alias melange)
+  >  (libraries dummyfoo)
+  >  (module_system es6))
+  > EOF
+
+  $ dune build @melange 2>&1 | awk '/Internal error/,/Raised/'
+  Internal error, please report upstream including the contents of _build/log.
+  Description:
+    ("Option.value_exn", {})
+  Raised at Stdune__Code_error.raise in file