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/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 1f52d9f..8a84312 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -3022,6 +3022,8 @@
 test_garbagecollect_now()	none	free memory right now for testing
 test_garbagecollect_soon()	none	free memory soon for testing
 test_getvalue({string})		any	get value of an internal variable
+test_gui_drop_files({list}, {row}, {col}, {mods})
+				none	drop a list of files in a window
 test_gui_mouse_event({button}, {row}, {col}, {repeated}, {mods})
 				none	add a mouse event to the input buffer
 test_ignore_error({expr})	none	ignore a specific error