Minor test improvements to support upcoming PHP 8

[clue]

This component now supports PHP 8 just fine 🎉

~/workspace/reactphp-dns$ $ docker run -it --rm --net host -v `pwd`:/data --workdir /data php:8.0.0beta2-cli-alpine php vendor/bin/phpunit
PHPUnit 9.3.8 by Sebastian Bergmann and contributors.

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Time: 00:01.558, Memory: 21.14 MB

OK (253 tests, 675 assertions)

This PR does not currently include PHP 8 in the Travis test matrix. PHP is scheduled to be released in November, I'll file a follow-up PR once it's available for installation on Travis.

Builds on top of #166, #165 and #163