patch 8.2.4259: number of test functions for GUI events is growing

Problem:    Number of test functions for GUI events is growing.
Solution:   Use one function with a dictionary. (Yegappan Lakshmanan,
            closes #9660)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 623fd3b..f1d88b9 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1117,10 +1117,7 @@
 	test_garbagecollect_now()   free memory right now
 	test_garbagecollect_soon()  set a flag to free memory soon
 	test_getvalue()		get value of an internal variable
-	test_gui_drop_files()	drop file(s) in a window
-	test_gui_mouse_event()	add a GUI mouse event to the input buffer
-	test_gui_tabline_event() add a GUI tabline event to the input buffer
-	test_gui_tabmenu_event() add a GUI tabmenu event to the input buffer
+	test_gui_event()	generate a GUI event for testing
 	test_ignore_error()	ignore a specific error message
 	test_null_blob()	return a null Blob
 	test_null_channel()	return a null Channel