test: reproduce dune exec -w crash with pkg management

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

test/blackbox-tests/test-cases/pkg/gh10959.t

@@ -0,0 +1,42 @@
+Repro `dune exec --watch` crash with pkg management
+
+  $ . ./helpers.sh
+
+  $ mkrepo
+  $ add_mock_repo_if_needed
+
+
+  $ mkdir _multiple
+  $ cat >_multiple/dune-project <<EOF
+  > (lang dune 3.13)
+  > (package (name foo) (allow_empty))
+  > (package (name bar) (allow_empty))
+  > EOF
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.13)
+  > (pin
+  >  (url file://$PWD/_multiple)
+  >  (package (name foo))
+  >  (package (name bar)))
+  > (package
+  >  (name main)
+  >  (depends foo bar))
+  > EOF
+
+  $ cat >dune <<EOF
+  > (executable
+  >  (name x))
+  > EOF
+  $ cat >x.ml <<EOF
+  > let () = print_endline "x"
+  > EOF
+
+  $ dune pkg lock
+  Solution for dune.lock:
+  - bar.dev
+  - foo.dev
+
+  $ dune exec -w ./x.exe 2>&1 | grep -io "I must not crash"
+  I must not crash
+