test(select): exercise (select ..) with (include_subdirs unqualified) (#13172)

reproduces #12450

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

Author: anmonteiro

test/blackbox-tests/test-cases/select-field/select-unqualified.t

@@ -0,0 +1,51 @@
+We test the `(select)` field of the `(libraries)` field in the presence of
+`(include_subdirs unqualified)`
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.22)
+  > EOF
+
+  $ cat > dune <<EOF
+  > (include_subdirs unqualified)
+  > (library
+  >  (name foo)
+  >  (libraries
+  >   (select bar.ml from
+  >    (unix -> bar.unix.ml)
+  >    (!unix -> bar.nounix.ml))))
+  > EOF
+
+  $ mkdir -p sub
+  $ cat > sub/bar.unix.ml <<EOF
+  > let () = print_endline "Test: Unix was found!"
+  > EOF
+  $ cat > sub/bar.nounix.ml <<EOF
+  > let () = print_endline "Test: Unix was not found!"
+  > EOF
+
+The select field does not pick up the module sources for the test stanza
+correctly. This is a bug.
+
+  $ dune build foo.cma
+  File "_unknown_", line 1, characters 0-0:
+  Error: No rule found for bar.unix.ml
+  [1]
+
+  $ cat > dune <<EOF
+  > (include_subdirs unqualified)
+  > (library
+  >  (name foo)
+  >  (libraries
+  >   (select bar.ml from
+  >    (unix -> sub/bar.unix.ml)
+  >    (!unix -> sub/bar.nounix.ml))))
+  > EOF
+
+It also doesn't allow specifying the path
+
+  $ dune build foo.cma
+  File "dune", line 6, characters 12-27:
+  6 |    (unix -> sub/bar.unix.ml)
+                  ^^^^^^^^^^^^^^^
+  Error: The format for files in this select branch must be bar.{name}.ml
+  [1]