Add test coverage for Tasks.php

Problem/Motivation

This issue is related to #2501835: Remove SafeMarkup::set in Drupal\Core\Database\Install\Tasks::runTasks() and ensure multiple messages are printed. While working on https://www.drupal.org/node/2501835, the bug was discovered in HEAD where Tasks.php throws only last database error during site installation instead of displaying all the failed database error messages. The below code has been added into #2501835-22: Remove SafeMarkup::set in Drupal\Core\Database\Install\Tasks::runTasks() and ensure multiple messages are printed.

+  if ($failure) {
+      $message = array(
+        'error' => array(
+          '#markup' => t('Resolve all issues below to continue the installation. For help configuring your database server, see the <a href="https://www.drupal.org/getting-started/install">installation handbook</a>, or contact your hosting provider.'),
+        ),
+        'item_list' => array(
+          '#theme' => 'item_list',
+          '#items' => $error,
+        ),
+      );
+      $message = \Drupal::service('renderer')->render(($message), 'error');

Proposed resolution

Add the test coverage for displaying multiple database error messages.

Remaining tasks

User interface changes

None

API changes

None

Data model changes