Add string type to protected $defaultTheme in test classes

Problem/Motivation

Follow-up from 🐛 Fix label token replacement for views entity reference arguments RTBC where @kim.pepper asked for a typehint on:

protected $defaultTheme = 'stark';

We don't yet do that anywhere in core (that I can see). If we're doing it, let's do it everywhere.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet