Skip mocking DNS resolver (warning)

> don't mock what you don't own

Our test suite currently tries to create mock of `React\Dns\Resolver\Resolver` which is now marked `final` as of https://github.com/reactphp/dns/pull/134. This leads to a "warning" as of PHP 5.6 and a build error in older PHP/PHPUnit versions. We should update the test suite of this component to no longer try to mock this class. Any input / PR is appreciated :+1: 