patch 8.2.3251: listing builtin_gui as an available terminal is confusing

Problem:    Listing builtin_gui as an available terminal is confusing.
Solution:   Do not list builtin_gui. (Christian Brabandt, closes #8669,
            closes #8661)
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index 2f2fe3b..f73770b 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -2348,4 +2348,16 @@
   bw!
 endfunc
 
+func Test_terminal_builtin_without_gui()
+  CheckNotMSWindows
+
+  " builtin_gui should not be output by :set term=xxx
+  let output = systemlist("TERM=dumb " .. v:progpath .. " --clean -c ':set t_ti= t_te=' -c 'set term=xxx' -c ':q!'")
+  redraw!
+  call map(output, {_, val -> trim(val)})
+  call assert_equal(-1, index(output, 'builtin_gui'))
+  call assert_notequal(-1, index(output, 'builtin_dumb'))
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab