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.
............................................................... 63 / 253 ( 24%)
............................................................... 126 / 253 ( 49%)
............................................................... 189 / 253 ( 74%)
............................................................... 252 / 253 ( 99%)
. 253 / 253 (100%)
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