patch 8.2.5084: when the GUI shows a dialog tests get stuck

Problem:    When the GUI shows a dialog tests get stuck.
Solution:   Add the --gui-dialog-file argument.

src/structs.h

@@ -4207,6 +4207,9 @@ typedef struct
 
     int		want_full_screen;
     int		not_a_term;		// no warning for missing term?
+#ifdef FEAT_GUI
+    char_u	*gui_dialog_file;	// file to write dialog text in
+#endif
     int		tty_fail;		// exit if not a tty
     char_u	*term;			// specified terminal name
 #ifdef FEAT_CRYPT