TestHttpClientMiddleware should only set User-Agent for Drupal requests

Actually, we can solve both problems here: explicitly throw an exception if we make a request to another host, and then we will never send the Drupal User-Agent to any other host anyway.

Nice catch!

Sounds like known bug 🐛 Tests should not do requests to updates.drupal.org Needs work

I imagine this is going to break contrib or custom tests that explicitly want to talk to third party services (remote search servers for example).

This is flushing out a lot of http requests in core tests too. I'm not sure we can commit it even once those are fixed for the reasons given in #7, maybe we can add some kind of flag to enable the behaviour or not?

OEmbedTestTrait has what might be a better method:

  /**
   * Configures the HTTP client to always use the fixtures directory.
   *
   * All requests are carried out relative to the URL of the fixtures directory.
   * For example, after calling this method, a request for foobar.html will
   * actually request http://test-site/path/to/fixtures/foobar.html.
   */
  protected function lockHttpClientToFixtures() {
    $this->writeSettings([
      'settings' => [
        'http_client_config' => [
          'base_uri' => (object) [
            'value' => $this->getFixturesUrl() . '/',
            'required' => TRUE,
          ],
        ],
      ],
    ]);

This is acting differently on local vs GitLab CI and I have no idea why.

Removing PHPUnit 10 tag, it's no longer strictly related to the PHPUnit upgrade.

If we block requests to anything other than the site under test we're going to break contrib and custom tests big time.

I think it would fine if we only blocked it for core tests, but it would be good to 100% enforce it in core because it is easy to do this without realising - as we've found out.