BrowserTestBase::$defaultTheme is required.

Problem/Motivation

The migrate_file module's tests are failing due to a missing `$defaultTheme` property in the test classes that extend `BrowserTestBase`. BTB based tests not specifying defaultTheme has been deprecated since Drupal 8.8. Will soldily break in D10 -- see changelog.

Steps to reproduce

1. Install the migrate_file module on a Drupal 9 or 10 setup.
2. Run PHPStan analysis on the module using the command: `php vendor/bin/phpstan analyse web/modules/contrib/migrate_file --level 2`.
3. Observe the error message indicating that `$defaultTheme` is required in `LoadTest.php`.

Proposed resolution

Add the following line to all test classes extending `BrowserTestBase`:

```php
protected $defaultTheme = 'stark';