Add test for duplicate pins

[Leonidas-from-XIV]

I noticed that there is no test that checked the "package %S is already defined" message:

$ rg "package .* is already defined"
src/dune_lang/dune_project.ml
513:          "package %s is already defined in %S"
813:      [ Pp.textf "package %s is already defined" (Package.name p |> Package.Name.to_string)

src/dune_lang/pin_stanza.ml
32:      [ Pp.textf "package %S is already defined" (Package_name.to_string name) ]

test/blackbox-tests/test-cases/pkg/pin-stanza/duplicate-pins.t
46:  Error: package "dep_a" is already defined

test/blackbox-tests/test-cases/exclusive-package-dirs.t
19:  Error: package foo is already defined in "foo"

So I wrote a test. It works properly. Thus I was curious what happens if I declare a duplicate pin via a dependency. It gets ignored (which come to think of it, is probably the right thing to do). See the test.

[ElectreAAS]

There is a slight overlap between this test and override-single-workspace.t, but I think that's fine