Fix failing test on PHP 7.4 with code coverage when skipping some tests

This only reports an error on PHP 7.4 when executed like this:
$ vendor/bin/phpunit --coverage-text --exclude-group internet

Author: clue

tests/FunctionalResolverTest.php

@@ -188,6 +188,7 @@ public function testCancelResolveShouldNotCauseGarbageReferences()
         $this->resolver = $factory->create('127.0.0.1', $this->loop);
 
         gc_collect_cycles();
+        gc_collect_cycles(); // clear twice to avoid leftovers in PHP 7.4 with ext-xdebug and code coverage turned on
 
         $promise = $this->resolver->resolve('google.com');
         $promise->cancel();