Fix Blocks tests that rely on UID1's super user behavior

Problem/Motivation

In #540008: Add a container parameter that can remove the special behavior of UID#1 an approach was taken where we can simply flag tests that are failing if we turn off user 1's super user powers, so that they can be taken care of in a followup. This issue is to collect all of these followups.

The goal is to have no tests in Drupal core that rely on UID1's special privileges so that we:

1. Know these tests are correctly assigning the necessary permissions to run
2. Can turn off the super user access policy in D11, knowing it won't break core
3. Can remove the super user access policy in D12, providing an admin account recovery tool to replace it

Steps to reproduce

Go into any of the tests flagged with:

  /**
   * {@inheritdoc}
   *
   * @todo Remove and fix test to not rely on super user.
   * @see https://www.drupal.org/project/drupal/issues/3437620
   */

And:

1. Remove the code below that sets the usesSuperUserAccessPolicy to TRUE.
2. Run the test to see which test methods are failing

Proposed resolution

Assign the right permissions to make the test go green without the super user access policy. Those few tests that specifically test said policy can obviously stay, but will be removed along with the policy in D12.

Remaining tasks

• core/modules/block/tests/src/
  • Functional/BlockHtmlTest.php
  • Functional/BlockXssTest.php
  • FunctionalJavascript/BlockContextualLinksTest.php