Fix patch expect tests on macos (#11132)

A recent update to macos's patch command broke some of our tests. The
relevant changes to the command's behaviour are that it's stricter about
line numbers in patch metadata, and it no longer deletes files,
truncating them instead. This updates dune's patch expect tests to
accomodate this behaviour.

Signed-off-by: Stephen Sherratt <[email protected]>

test/expect-tests/dune_patch/dune_patch_tests.ml

@@ -9,7 +9,7 @@ diff --git a/foo.ml b/foo.ml
 index b69a69a5a..ea988f6bd 100644
 --- a/foo.ml
 +++ b/foo.ml
-@@ -1,2 +1,2 @@
+@@ -1,1 +1,1 @@
 -This is wrong
 +This is right
 |}
@@ -22,7 +22,7 @@ diff --git a/dir/foo.ml b/dir/foo.ml
 index b69a69a5a..ea988f6bd 100644
 --- a/dir/foo.ml
 +++ b/dir/foo.ml
-@@ -1,2 +1,2 @@
+@@ -1,1 +1,1 @@
 -This is wrong
 +This is right
 |}
@@ -199,10 +199,21 @@ let%expect_test "patching a new file" =
 let () = Dune_util.Report_error.report_backtraces true
 
 let%expect_test "patching a deleted file" =
-  test [ "foo.ml", "This is wrong\n" ] ("foo.patch", delete_file);
-  check "foo.ml";
-  [%expect {|
-    File foo.ml not found |}]
+  let filename = "foo.ml" in
+  test [ filename, "This is wrong\n" ] ("foo.patch", delete_file);
+  (* Different implementations of the patch command behave differently when the
+     patch specifies deleting a file: *)
+  match Unix.stat filename with
+  | { st_kind = S_REG; st_size; _ } ->
+    (* Some implementations of patch (e.g. the patch that ships with macOS
+       15.1) do not delete files, and instead truncate them to 0 length. If
+       the file still exists after applying the patch, assert that it is now
+       empty. *)
+    assert (st_size == 0)
+  | _ -> failwith "Not a regular file"
+  | exception Unix.Unix_error (Unix.ENOENT, _, _) ->
+    (* Most implementations of patch will delete the file. *)
+    ()
 ;;
 
 let undo_breaks =