luci-base: ui.js: fix UICheckbox widget operation when tooltips are set

When a tooltip is rendered for a checkbox widget, an additional node is
placed after the checkbox label element, breaking DOM selectors in
bind(), isChecked(), setValue().

Apparently the functionality was never actually tested.

Fixes: #4938
Fixes: e951236e

 ("luci-base: add tooltip handling")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>