Fix SelectionButton tests for jQuery 1.11.3
Fix SelectionButton tests for jQuery 1.11.3

* Checkbox and radio states should be set using prop rather than attr
* Trigger reliably changes the state of a checkbox, we don’t need to
set the attr as well – doing both is confusing and might achieve the
opposite of what’s intended

alphagov/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,alphagov/govuk_frontend_toolkit,quis/govuk_frontend_toolkit,quis/govuk_frontend_toolkit