WebUI Tests: fixing test_user.py::test_test_noprivate_posix

When filling the combo box (the gidnumber) in the dialog to create a new
user, the Add button was also clicked; closing the dialog. The wait
makes it to not click.

Reviewed-By: Petr Vobornik <pvoborni@redhat.com>

ipatests/test_webui/ui_driver.py

@@ -858,6 +858,7 @@ class UI_driver(object):
         if combobox_input:
             if not option:
                 self.fill_textbox(combobox_input, value, cb)
+                open_btn.click()
         else:
             if not option:
                 # try to search