patch 8.2.4276: separate test function for the GUI scrollbar

Problem:    Separate test function for the GUI scrollbar.
Solution:   Use test_gui_event(). (Yegappan Lakshmanan, closes #9674)

runtime/doc/builtin.txt

@@ -654,8 +654,6 @@ test_null_string()		String	null value for testing
 test_option_not_set({name})	none	reset flag indicating option was set
 test_override({expr}, {val})	none	test with Vim internal overrides
 test_refcount({expr})		Number	get the reference count of {expr}
-test_scrollbar({which}, {value}, {dragging})
-				none	scroll in the GUI for testing
 test_setmouse({row}, {col})	none	set the mouse position for testing
 test_settime({expr})		none	set current time for testing
 test_srand_seed([seed])		none	set seed for testing srand()

runtime/doc/usr_41.txt

@@ -1130,7 +1130,6 @@ Testing:				    *test-functions*
 	test_setmouse()		set the mouse position
 	test_feedinput()	add key sequence to input buffer
 	test_option_not_set()	reset flag indicating option was set
-	test_scrollbar()	simulate scrollbar movement in the GUI
 	test_refcount()		return an expression's reference count
 	test_srand_seed()	set the seed value for srand()
 	test_unknown()		return a value with unknown type