patch 8.2.0299: Vim9: ISN_STORE with argument not tested

Problem:    Vim9: ISN_STORE with argument not tested.  Some cases in tv2bool()
            not tested.
Solution:   Add tests.  Add test_unknown() and test_void().
diff --git a/src/proto/testing.pro b/src/proto/testing.pro
index 2b001a5..7542ccb 100644
--- a/src/proto/testing.pro
+++ b/src/proto/testing.pro
@@ -28,6 +28,8 @@
 void f_test_null_list(typval_T *argvars, typval_T *rettv);
 void f_test_null_partial(typval_T *argvars, typval_T *rettv);
 void f_test_null_string(typval_T *argvars, typval_T *rettv);
+void f_test_unknown(typval_T *argvars, typval_T *rettv);
+void f_test_void(typval_T *argvars, typval_T *rettv);
 void f_test_scrollbar(typval_T *argvars, typval_T *rettv);
 void f_test_setmouse(typval_T *argvars, typval_T *rettv);
 void f_test_settime(typval_T *argvars, typval_T *rettv);