Automated tests are failing with Drupal Core 8.8 and later due to missing defaultTheme

Problem/Motivation

Automated tests are failing with Drupal Core 8.8 and later with a deprecation notice:

Drupal\Tests\BrowserTestBase::$defaultTheme is required in drupal:9.0.0 when using an install profile that does not set a default theme. Please see https://www.drupal.org/node/3083055

So, please add to each test class the following property:

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

Proposed resolution

Implement a $defaultTheme for the functional tests to resolve the deprecation notice.

Remaining tasks

• Write a patch
• Review
• Commit

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.