patch 8.2.4157: terminal test fails because Windows sets the title

Problem:    Terminal test fails because Windows sets the title.
Solution:   Add the "vterm_title" testing override and use it in the test.
            (Ozaki Kiichi, closes #9556)

runtime/doc/testing.txt

@@ -208,6 +208,8 @@ test_override({name}, {val})				*test_override()*
 		ui_delay     time in msec to use in ui_delay(); overrules a
 			     wait time of up to 3 seconds for messages
 		uptime 	     overrules sysinfo.uptime
+		vterm_title  setting the window title by a job running in a
+			     terminal window
 		ALL	     clear all overrides ({val} is not used)
 
 		"starting" is to be used when a test should behave like