pkg: add repro for gh10991 (ocaml#10993)

Signed-off-by: Stephen Sherratt <stephen@sherra.tt>

test/blackbox-tests/test-cases/pkg/ocamlformat/gh10991.t

@@ -0,0 +1,50 @@
+Test that ocamlformat is re-run when a source file changes.
+
+  $ . ./helpers.sh
+  $ mkrepo
+  $ make_project_with_dev_tool_lockdir
+
+Make a fake ocamlformat package that appends a comment to the end of its input.
+  $ mkpkg ocamlformat <<EOF
+  > install: [
+  >   [ "sh" "-c" "echo '#!/bin/sh' > %{bin}%/ocamlformat" ]
+  >   [ "sh" "-c" "echo 'cat \$2' >> %{bin}%/ocamlformat" ]
+  >   [ "sh" "-c" "echo 'echo \$2 | grep .*.ml >/dev/null && echo \"(* formatted with fake ocamlformat *)\"' >> %{bin}%/ocamlformat" ]
+  >   [ "sh" "-c" "chmod a+x %{bin}%/ocamlformat" ]
+  > ]
+  > EOF
+
+Initial file:
+  $ cat foo.ml
+  let () = print_endline "Hello, world"
+
+  $ DUNE_CONFIG__LOCK_DEV_TOOL=enabled dune fmt
+  Solution for dev-tools.locks/ocamlformat:
+  - ocamlformat.0.0.1
+  File "foo.ml", line 1, characters 0-0:
+  Error: Files _build/default/foo.ml and _build/default/.formatted/foo.ml
+  differ.
+  Promoting _build/default/.formatted/foo.ml to foo.ml.
+  [1]
+
+After formatting the fake ocamlformat has added a suffix:
+  $ cat foo.ml
+  let () = print_endline "Hello, world"
+  (* formatted with fake ocamlformat *)
+
+Update the file:
+  $ cat > foo.ml <<EOF
+  > let () = print_endline "Hello, ocaml!"
+  > EOF
+
+  $ DUNE_CONFIG__LOCK_DEV_TOOL=enabled dune fmt
+  File "foo.ml", line 1, characters 0-0:
+  Error: Files _build/default/foo.ml and _build/default/.formatted/foo.ml
+  differ.
+  Promoting _build/default/.formatted/foo.ml to foo.ml.
+  [1]
+
+After formatting a second time, the recent change to the file was ignored:
+  $ cat foo.ml
+  let () = print_endline "Hello, world"
+  (* formatted with fake ocamlformat *)