patch 9.0.1084: code handling low level MS-Windows events cannot be tested

Problem:    Code handling low level MS-Windows events cannot be tested.
Solution:   Add test_mswin_event() and tests using it. (Christopher Plewright,
            closes #11622)
diff --git a/src/proto/testing.pro b/src/proto/testing.pro
index 2192e91..dea4f75 100644
--- a/src/proto/testing.pro
+++ b/src/proto/testing.pro
@@ -33,6 +33,7 @@
 void f_test_unknown(typval_T *argvars, typval_T *rettv);
 void f_test_void(typval_T *argvars, typval_T *rettv);
 void f_test_setmouse(typval_T *argvars, typval_T *rettv);
+void f_test_mswin_event(typval_T *argvars, typval_T *rettv);
 void f_test_gui_event(typval_T *argvars, typval_T *rettv);
 void f_test_settime(typval_T *argvars, typval_T *rettv);
 /* vim: set ft=c : */