Bump wyrihaximus/async-test-utilities from 3.4.18 to 3.4.20

Bumps [wyrihaximus/async-test-utilities](https://github.com/WyriHaximus/php-async-test-utilities) from 3.4.18 to 3.4.20.
- [Release notes](https://github.com/WyriHaximus/php-async-test-utilities/releases)
- [Commits](WyriHaximus/php-async-test-utilities@3.4.18...3.4.20)

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Author: dependabot[bot]