Specify the $defaultTheme property in all functional tests

Problem/Motivation

#3082655: Specify the $defaultTheme property in all functional tests just landed in 8.8.x. Yay! This is actually a Good Thing(tm). ;)
But it's going to break our 8.8.x tests again now that #3090175: Let tests fail on deprecation notices is in. ;)

Proposed resolution

Explicitly define this in all our Functional and FunctionalJavascript tests:

  /**
   * {@inheritdoc}
   */
  protected $defaultTheme = 'stark';

If any tests break b/c of it, we can use 'classy' instead. We can also consider immediately fixing those test assertions to not rely on classy markup (hopefully this won't be too big a job) as part of this issue.

Thankfully, this is available in 8.7.x, so this won't add to the list at #3093193: Require Drupal 8.8

Remaining tasks

1. Do it.
2. See if any of our tests are depending on classy markup.
3. Decide what to do with them (fix here vs. set defaultTheme to classy and open followups)
4. Commit

User interface changes

API changes

Data model changes

Release notes snippet