Add -- test.

tests/dashdash_test.toit

@@ -0,0 +1,21 @@
+// Copyright (C) 2024 Toitware ApS.
+// Use of this source code is governed by a Zero-Clause BSD license that can
+// be found in the tests/LICENSE file.
+
+import cli
+import expect show *
+
+main:
+  root := cli.Command "root"
+    --rest=[
+      cli.Option "first" --required,
+      cli.Option "arg" --multi
+    ]
+    --run=:: | _ parsed| test-dashdash parsed
+  root.run ["--", "prog", "arg1", "arg2", "arg3"]
+
+test-dashdash parsed/cli.Parsed:
+  first := parsed["first"]
+  rest := parsed["arg"]
+  expect-equals "prog" first
+  expect-list-equals ["arg1", "arg2", "arg3"] rest