Investigate need of matthiasnoback/phpunit-asynchronicity to replace sleep() in tests

Problem/Motivation

While working on tests ontests/src/Kernel/ElasticSearchBackendTest.php the same approach as search_api_opensearch is using (https://git.drupalcode.org/project/search_api_opensearch/-/blob/2.x/test...), does not work. During the development I choose to use matthiasnoback/phpunit-asynchronicity library to avoid the issues it had (https://git.drupalcode.org/project/elasticsearch_connector/-/jobs/965367). Maybe communicate with search_api_opensearch project and figure out a solid fix for this.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes