Use BrowserKit with Guzzle client

[claudiu-cristea]

Problem

• We've switched from Goutte to BrowserKit driver in #620
• However, with BrowserKit, which uses the HttpBrowser, the forms that are posting files are failing. The reason is this https://github.com/symfony/symfony/issues/47504
• This issue could not be reproduced by Drupal Test Traits because they are using BrowserKit with Guzzle as client, via Drupal\Tests\DrupalTestBrowser, see https://gitlab.com/weitzman/drupal-test-traits/-/blob/2.x/src/BrowserKitTrait.php#L21

Proving the bug

I've created a "test only" PR #627

Proposal

Use Drupal\Tests\DrupalTestBrowser as BrowserKitDriver client. This also has the advantage that is aligned with Drupal core PHPUnit tests.

[jhedstrom]

This looks great, thanks!