vim-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 vim/vim#1971)
9f0139a2a8

src/nvim/testdir/shared.vim

@@ -246,3 +246,7 @@ func! Screenline(lnum)
   let line = join(chars, '')
   return matchstr(line, '^.\{-}\ze\s*$')
 endfunc
+
+func CanRunGui()
+  return has('gui') && ($DISPLAY != "" || has('gui_running'))
+endfunc