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)

src/testdir/shared.vim

@@ -215,3 +215,7 @@ func RunVimPiped(before, after, arguments, pipecmd)
   endif
   return 1
 endfunc
+
+func CanRunGui()
+  return has('gui') && ($DISPLAY != "" || has('gui_running'))
+endfunc

src/testdir/test_gui.vim

@@ -1,6 +1,7 @@
 " Tests specifically for the GUI
 
-if !has('gui') || ($DISPLAY == "" && !has('gui_running'))
+source shared.vim
+if !CanRunGui()
   finish
 endif
 

src/testdir/test_gui_init.vim

@@ -1,7 +1,8 @@
 " Tests specifically for the GUI features/options that need to be set up at
 " startup to take effect at runtime.
 
-if !has('gui') || ($DISPLAY == "" && !has('gui_running'))
+source shared.vim
+if !CanRunGui()
   finish
 endif
 

src/testdir/test_terminal.vim

@@ -417,7 +417,7 @@ endfunc
 
 " 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

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    933,
 /**/
     932,
 /**/