patch 8.0.0933: terminal test tries to start GUI when it's not possible

Problem:    Terminal test tries to start GUI when it's not possible.
Solution:   Check if the GUI can run. (James McCoy, closes #1971)
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 4278351..605292e 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -417,7 +417,7 @@
 
 " must be last, we can't go back from GUI to terminal
 func Test_zz_terminal_in_gui()
-  if !has('gui')
+  if !CanRunGui()
     return
   endif
   gui -f