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/builtin.txt b/runtime/doc/builtin.txt
index aaf4e3b..33bb907 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -642,13 +642,7 @@
 test_garbagecollect_now()	none	free memory right now for testing
 test_garbagecollect_soon()	none	free memory soon for testing
 test_getvalue({string})		any	get value of an internal variable
-test_gui_drop_files({list}, {row}, {col}, {mods})
-				none	drop a list of files in a window
-test_gui_mouse_event({button}, {row}, {col}, {repeated}, {mods})
-				none	add a mouse event to the input buffer
-test_gui_tabline_event({tabnr}) Bool	add a tabline event to the input buffer
-test_gui_tabmenu_event({tabnr}, {event})
-				none	add a tabmenu event to the input buffer
+test_gui_event({event}, {args})	bool	generate a GUI event for testing
 test_ignore_error({expr})	none	ignore a specific error
 test_null_blob()		Blob	null value for testing
 test_null_channel()		Channel	null value for testing