Skip affected test cases to avoid bogus test failure on PHP 7.4

The build error on PHP 7.4 will be addressed via
reactphp/dns#164. Interestingly, it fails in the
`FunctionalResolverTest` class and does not fail anymore by excluding
some other test cases. It can only be reproduced by running:

```
$ vendor/bin/phpunit --coverage-text --exclude-group internet
```

It can not be reproduced when running on any other PHP version, when
`ext-xdebug` is disabled, when coverage is not collected or when not
excluding this group.

This means the test failure is bogus.

We work around this by temporarily skipping some test case for now.

Author: clue

phpunit.xml.dist

@@ -4,6 +4,10 @@
     <testsuites>
         <testsuite name="ReactPHP Test Suite">
             <directory>./vendor/react/*/tests/</directory>
+            <!-- temporarily skip broken tests, see https://github.com/reactphp/dns/pull/164 -->
+            <exclude>./vendor/react/dns/tests/FunctionalResolverTest.php</exclude>
+            <exclude>./vendor/react/dns/tests/Query/CoopExecutorTest.php</exclude>
+            <exclude>./vendor/react/dns/tests/Query/SelectiveTransportExecutorTest.php</exclude>
         </testsuite>
     </testsuites>