patch 8.2.3040: GUI: dropping files not tested

Problem:    GUI: dropping files not tested.
Solution:   Add test_gui_drop_files() and tests. (Yegappan Lakshmanan,
            closes #8434)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 21d28af..3dd887b 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -1700,6 +1700,8 @@
 			ret_void,	    f_test_garbagecollect_soon},
     {"test_getvalue",	1, 1, FEARG_1,	    NULL,
 			ret_number,	    f_test_getvalue},
+    {"test_gui_drop_files",	4, 4, 0,	    NULL,
+			ret_void,	    f_test_gui_drop_files},
     {"test_gui_mouse_event",	5, 5, 0,	    NULL,
 			ret_void,	    f_test_gui_mouse_event},
     {"test_ignore_error", 1, 1, FEARG_1,    NULL,