Add tests for the autocomplete widget

Problem/Motivation

There are no tests for the autocomplete widget.

As noted in #3330764: Add 'title' to field config to use as default for widget config, this needs to use FunctionalJavascript to test; I didn't want to hold that issue up whilst that was set up locally, hence this follow up.

Steps to reproduce

N/A

Proposed resolution

Add Functional tests.

Remaining tasks

All the things.

User interface changes

None.

API changes

None.

Data model changes

None.