Issue #3280882 by mondrake, mallezie: KernelTestBase::tearDown() cleanup...

    $this->vfsRoot = NULL;

    $this->configImporter = NULL;

    // Free up memory: Custom test class properties.

    // Note: Private properties cannot be cleaned up.

    $rc = new \ReflectionClass(__CLASS__);

    $blacklist = [];

    foreach ($rc->getProperties() as $property) {

      $blacklist[$property->name] = $property->getDeclaringClass()->name;

    }

    $rc = new \ReflectionClass($this);

    foreach ($rc->getProperties(\ReflectionProperty::IS_PUBLIC | \ReflectionProperty::IS_PROTECTED) as $property) {

      if (!$property->isStatic() && !isset($blacklist[$property->name])) {

        $this->{$property->name} = NULL;

      }

    }

    // Clean FileCache cache.

    FileCache::reset();