test: show limitation installing multi-context libs (#10493)

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

test/blackbox-tests/test-cases/enabled_if/install-multiple-context-libs.t

@@ -0,0 +1,48 @@
+Test installation for melange libraries in multiple contexts.
+The expectation is that libraries that don't collide (e.g. a melange and a
+native library) can be installed in the same context.
+
+Create a package named foo
+
+  $ mkdir -p a b
+  $ cat > dune-project << EOF
+  > (lang dune 3.13)
+  > (using melange 0.1)
+  > (package (name foo))
+  > EOF
+
+Define libraries in multiple contexts
+
+  $ cat > dune-workspace << EOF
+  > (lang dune 3.13)
+  > (context default)
+  > (context
+  >  (default
+  >   (name melange)))
+  > EOF
+  $ cat > a/dune << EOF
+  > (library
+  >  (name foo)
+  >  (public_name foo.native)
+  >  (enabled_if (= %{context_name} "default")))
+  > (library
+  >  (name foo)
+  >  (public_name foo.melange)
+  >  (modes melange)
+  >  (enabled_if (= %{context_name} "melange")))
+  > EOF
+  $ cat > a/foo.ml <<EOF
+  > let () = print_endline "foo"
+  > EOF
+
+  $ dune build
+
+  $ mkdir -p out/man
+
+
+Try installing both
+
+  $ dune install foo --prefix out
+  Error: Cannot specify '--prefix' or '--libdir' when installing into multiple
+  contexts!
+  [1]