patch 9.0.0400: GUI test sometimes hangs on CI

Problem:    GUI test sometimes hangs on CI.
Solution:   Delete a test file explicitly. (Ken Takata, closes #11072)

src/testdir/test_gui.vim

@@ -1607,12 +1607,15 @@ func Test_gui_CTRL_SHIFT_V()
 endfunc
 
 func Test_gui_dialog_file()
+  " make sure the file does not exist, otherwise a dialog makes Vim hang
+  call delete('Xdialfile')
+
   let lines =<< trim END
     file Xdialfile
     normal axxx
     confirm qa
   END
-  call writefile(lines, 'Xlines')
+  call writefile(lines, 'Xlines', 'D')
   let prefix = '!'
   if has('win32')
     let prefix = '!start '
@@ -1624,7 +1627,6 @@ func Test_gui_dialog_file()
 
   call delete('Xdialog')
   call delete('Xdialfile')
-  call delete('Xlines')
 endfunc
 
 " Test for sending low level key presses

src/version.c

@@ -703,6 +703,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    400,
 /**/
     399,
 /**/