Remove Class Constant Visibility from HelpTopicTwigTest which breaks tests at PHP 7

Problem/Motivation

Follow-up of #3037228: Add more test coverage to Help Topics.

Class Constant Visibility support was added at PHP 7.1.
PHP 7.1.0 Released
PHP RFC: Support Class Constant Visibility

The new test HelpTopicTwigTest fails at PHP 7 (8.8.x-dev test with PHP 7 & MySQL 5.5).

Unknown
fail: [run-tests.sh check] Line 0 of :
FATAL Drupal\Tests\help_topics\Unit\HelpTopicTwigTest: test runner returned a non-zero error code (255).

https://www.drupal.org/pift-ci-job/1422958
https://www.drupal.org/pift-ci-job/1423765

Proposed resolution

--- a/core/modules/help_topics/tests/src/Unit/HelpTopicTwigTest.php
+++ b/core/modules/help_topics/tests/src/Unit/HelpTopicTwigTest.php
@@ -30,7 +30,7 @@ class HelpTopicTwigTest extends UnitTestCase {
    *
    * @var array
    */
-  protected const PLUGIN_INFORMATION = [
+  const PLUGIN_INFORMATION = [
     'id' => 'test.topic',
     'provider' => 'test',
     'label' => 'This is the topic label',