[D7 PHP 8.3] unserialize(): Extra data starting at offset

Problem/Motivation

There are unserialization errors in PHP 8.3 test suite on D7, see: https://www.drupal.org/pift-ci-job/2739137

exception: [Warning] Line 2193 of modules/simpletest/drupal_web_test_case.php:
unserialize(): Extra data starting at offset 243 of 244 bytes

exception: [Warning] Line 2193 of modules/simpletest/drupal_web_test_case.php:
unserialize(): Extra data starting at offset 251 of 252 bytes

exception: [Warning] Line 2193 of modules/simpletest/drupal_web_test_case.php:
unserialize(): Extra data starting at offset 213 of 214 bytes

These errors are causing the DrupalErrorCollectionUnitTest::testErrorCollect() test to fail, because the code expects three manually triggered messages.

Steps to reproduce

Run D7 core tests on PHP 8.3.

Proposed resolution

Fix the problem.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet