This groups tests into common HTML files so they can be run more
quickly. This also reduces the number of Sauce Labs instances that
have to be run, as one instance was previously spun up for each
file.
In the past, Select2 triggered the `select2-clearing` event when
clearing the placeholder. This has been switched to the `unselect`
event which is now triggered whenever the placeholder is cleared.
The placeholder can also be prevented if the `unselecting` event
is prevented.
This will also trigger the event when deleting every option from
the multiple select, so it is possible for the `unselecting` event
to be triggered multiple times when clearing a multiple select.
This adds tests to ensure that the `allowClear` option always works.
This closes https://github.com/select2/select2/issues/2954.
