patch 8.2.4210: window title test fails in some configurations

Problem:    Window title test fails in some configurations.
Solution:   Only run the test if the title can be obtained.
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 3f197ca..39110bb 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -1632,6 +1632,9 @@
 
 func Test_setbufvar_keep_window_title()
   CheckRunVimInTerminal
+  if !has('title') || empty(&t_ts)
+    throw "Skipped: can't get/set title"
+  endif
 
   let lines =<< trim END
       edit Xa.txt