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/os_win32.pro b/src/proto/os_win32.pro
index a6bbd33..9f8b969 100644
--- a/src/proto/os_win32.pro
+++ b/src/proto/os_win32.pro
@@ -9,6 +9,7 @@
 void PlatformId(void);
 void mch_setmouse(int on);
 void mch_bevalterm_changed(void);
+int test_mswin_event(char_u *event, dict_T *args);
 void mch_update_cursor(void);
 int mch_char_avail(void);
 int mch_check_messages(void);