test: demonstrate @unused-libs issues with (modes melange) (#12826)

Signed-off-by: Antonio Nuno Monteiro <anmonteiro@gmail.com>

Author: anmonteiro

test/blackbox-tests/test-cases/melange/unused-libs-melange.t

@@ -0,0 +1,65 @@
+Demonstrate unused-libs issues with `(modes melange)`
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.21)
+  > (using melange 1.0)
+  > EOF
+
+  $ cat > dune <<EOF
+  > (library
+  >  (name used_lib)
+  >  (modes melange)
+  >  (modules used_lib))
+  > 
+  > (library
+  >  (name unused_lib)
+  >  (modes melange)
+  >  (modules unused_lib))
+  > 
+  > (library
+  >  (name unused_lib2)
+  >  (modes melange)
+  >  (modules unused_lib2))
+  > 
+  > (library
+  >  (name mylib)
+  >  (modules mylib)
+  >  (modes melange)
+  >  (libraries used_lib unused_lib unused_lib2))
+  > EOF
+
+  $ cat > used_lib.ml <<EOF
+  > let helper x = x + 1
+  > EOF
+
+  $ cat > unused_lib.ml <<EOF
+  > let unused_helper x = x * 2
+  > EOF
+
+  $ cat > unused_lib2.ml <<EOF
+  > let unused_helper x = x * 3
+  > EOF
+
+  $ cat > mylib.ml <<EOF
+  > let compute x = Used_lib.helper x
+  > EOF
+
+  $ dune build @unused-libs
+  Error: No rule found for .mylib.objs/byte/mylib.cmi
+  -> required by alias unused-libs in dune:16
+  Error: No rule found for .mylib.objs/byte/mylib.cmo
+  -> required by alias unused-libs in dune:16
+  Error: No rule found for .unused_lib.objs/byte/unused_lib.cmi
+  -> required by alias unused-libs in dune:6
+  Error: No rule found for .unused_lib.objs/byte/unused_lib.cmo
+  -> required by alias unused-libs in dune:6
+  Error: No rule found for .unused_lib2.objs/byte/unused_lib2.cmi
+  -> required by alias unused-libs in dune:11
+  Error: No rule found for .unused_lib2.objs/byte/unused_lib2.cmo
+  -> required by alias unused-libs in dune:11
+  Error: No rule found for .used_lib.objs/byte/used_lib.cmi
+  -> required by alias unused-libs in dune:1
+  Error: No rule found for .used_lib.objs/byte/used_lib.cmo
+  -> required by alias unused-libs in dune:1
+  [1]
+