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/gui.c b/src/gui.c
index b5ec007..c748fae 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -5567,6 +5567,7 @@
 	{
 	    vim_free(fnames[0]);
 	    vim_free(fnames);
+	    vim_free(p);
 	}
 	else
 	    handle_drop(count, fnames, (modifiers & MOUSE_CTRL) != 0,