Allow partial spoofing of the user agent during web/browser tests

I'm working on a module that has some business logic around the user agent, but I cannot write functional tests for this because changing the user agent breaks the drupal_valid_test_ua function which validates if a Drupal request originates from a test.

BrowserTestBase has this code that is called before each request:

  protected function prepareRequest() {
    $session = $this->getSession();
    $session->setCookie('SIMPLETEST_USER_AGENT', drupal_generate_test_ua($this->databasePrefix));
  }

Sprinkled throughout core are calls to drupal_valid_test_ua which has a regex for checking the user agent is valid for a test:

preg_match("/^simple(\w+\d+):(.+):(.+):(.+)$/", $user_agent, $matches)

I propose that the regex be changed to something like this, so other user agent text can be added to the end, separated by a space:

preg_match("/^simple(\w+\d+):(.+):(.+):([^ ]+)/", $user_agent, $matches)