WebTestBase::$root_user does not have a uid

Problem

• WebTestBase::setUp() installs Drupal through the interactive installer, and creates a fake user object for uid 1 and makes that available as $this->root_user, but the object does not contain a uid.
• The lack of a uid breaks WebTestBase::drupalCreateNode(), which tries to use the uid of the currently logged in user (which may be $this->root_user).

Proposed solution

1. Just add a ::$uid property with value 1 there.
2. No need to (re)load the entire entity of uid 1 (performance). It's OK if $this->root_user is a mock stdClass object.
3. No need for adding tests for this.