Add cram tests on the effect of optional arguments

plugins/qcheck-stm/test/test_optional_arguments.t

@@ -0,0 +1,32 @@
+In this file, we test the effect of the optional arguments of the command-line.
+
+  $ export ORTAC_ONLY_PLUGIN=qcheck-stm
+  $ cat > foo.mli << EOF
+  > type 'a t
+  > (*@ mutable model value : 'a *)
+  > val make : 'a -> 'a t
+  > (*@ t = make a
+  >     ensures t.value = a *)
+  > EOF
+  $ ortac qcheck-stm foo.mli "make 42" "int t" --include=mod | head
+  (* This file is generated by ortac qcheck-stm,
+     edit how you run the tool instead *)
+  open Foo
+  module Spec =
+    struct
+      open STM
+      [@@@ocaml.warning "-26-27"]
+      include Mod
+      type sut = int t
+      type cmd = |
+  $ ortac qcheck-stm foo.mli "make 42" "int t" --protect-call=protect | tail
+        match (cmd__004_, res__006_) with | _ -> true
+      let run cmd__010_ sut__011_ = match cmd__010_ with
+    end
+  module STMTests = (STM_sequential.Make)(Spec)
+  let _ =
+    Spec.protect
+      (fun () ->
+         QCheck_base_runner.run_tests_main
+           (let count = 1000 in
+            [STMTests.agree_test ~count ~name:"Foo STM tests"]))