patch 8.2.0763: GUI test fails without the terminal feature

Problem:    GUI test fails without the terminal feature.
Solution:   Check the terminal feature is supported. (Ken Takata,
            closes #6084)
diff --git a/src/testdir/test_gui.vim b/src/testdir/test_gui.vim
index 64b24bf..28782f0 100644
--- a/src/testdir/test_gui.vim
+++ b/src/testdir/test_gui.vim
@@ -841,6 +841,7 @@
 " Test for "!" option in 'guioptions'. Use a terminal for running external
 " commands
 func Test_gui_run_cmd_in_terminal()
+  CheckFeature terminal
   let save_guioptions = &guioptions
   set guioptions+=!
   if has('win32')