Skip mocking DNS resolver (warning)

[clue]

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 reactphp/dns#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 👍