Fix module test to use ptrdiff_t nargs too

Fix module test to use ptrdiff_t nargs too

* modules/mod-test/mod-test.c (Fmod_test_return_t)
(Fmod_test_sum, Fmod_test_signal, Fmod_test_throw)
(Fmod_test_non_local_exit_funcall, Fmod_test_globref_make)
(Fmod_test_string_a_to_b, Fmod_test_userptr_make)
(Fmod_test_userptr_get, Fmod_test_vector_fill)
(Fmod_test_vector_eq): Arg counts are ptrdiff_t, not int.
(finalizer): Remove; no longer used.