vim-patch:8.1.1831: confusing skipped message

Problem:    Confusing skipped message.
Solution:   Drop "run" from "run start the GUI".
2514315fc2

Cherry-pick 'CheckCanRunGui' user command from patch 8.1.1826.

src/nvim/testdir/check.vim

@@ -57,3 +57,11 @@ func CheckRunVimInTerminal()
     throw 'Skipped: cannot run Vim in a terminal window'
   endif
 endfunc
+
+" Command to check that we can run the GUI
+command CheckCanRunGui call CheckCanRunGui()
+func CheckCanRunGui()
+  if !has('gui') || ($DISPLAY == "" && !has('gui_running'))
+    throw 'Skipped: cannot start the GUI'
+  endif
+endfunc

src/nvim/testdir/shared.vim

@@ -329,7 +329,3 @@ func RunVimPiped(before, after, arguments, pipecmd)
   endif
   return 1
 endfunc
-
-func CanRunGui()
-  return has('gui') && ($DISPLAY != "" || has('gui_running'))
-endfunc