Add tests for including the custom generators

plugins/qcheck-stm/test/array_config.ml

@@ -3,3 +3,7 @@ open Array
 let init_sut = make 16 'a'
 
 type sut = char t
+
+module Gen = struct
+  let int = small_signed_int
+end

plugins/qcheck-stm/test/array_stm_tests.expected.ml

@@ -6,6 +6,12 @@ module Ortac_runtime = Ortac_runtime_qcheck_stm
 module Spec =
   struct
     open STM
+    module QCheck =
+      struct
+        include QCheck
+        module Gen = struct include Gen
+                            let int = small_signed_int end
+      end
     type sut = char t
     type cmd =
       | Length 

plugins/qcheck-stm/test/test_errors.t

@@ -250,3 +250,21 @@ We shouldn't be able to define a model by itsef in the `make` function:
                             ^
   Warning: Skipping clause: impossible to define the initial value of the model
            with a recursive expression.
+
+If we add some custom generators, we should do so in a `Gen` module that is implemented (functor application or reference to another module are not supported):
+  $ cat > foo.mli << EOF
+  > type 'a t
+  > (*@ mutable model value : 'a *)
+  > val make : 'a -> 'a t
+  > (*@ t = make a
+  >     ensures t.value = a *)
+  > EOF
+  $ cat > foo_config.ml << EOF
+  > module Gen = QCheck.Gen
+  > open Foo
+  > let init_sut = make 42 73
+  > type sut = int t
+  > EOF
+  $ ortac qcheck-stm foo.mli foo_config.ml
+  Error: Unsupported Gen module definition: only structures are allowed as
+         module definition here.