Add defaultTheme in tests extending BrowserTestBase.

Problem/Motivation

We should add defaultTheme where it will be required in tests extending BrowserTestBase.

Drupal\Tests\BrowserTestBase::$defaultTheme is required in drupal:9.0.0 when using an install profile that does not set a default theme. See https://www.drupal.org/node/2352949, which includes recommendations on which theme to use.

See https://www.drupal.org/node/3083055
and https://www.drupal.org/project/drupal/issues/3082655

Proposed resolution

Add the defaultTheme property to tests that extend BrowserTestBase, where appropriate or another theme where appropriate, based on https://www.drupal.org/node/3083055

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet