code: def test_layer_names(self):
      self.assertEqual(self.link.layer_names, ["l1", "f1", "f2", "l2"])
  
imports: import numpy
  import unittest
  import chainer
  from chainer.backends.cuda import to_cpu
  from chainer.function import Function
  from chainer import testing
  from chainer.testing import attr
  from chainercv.links import PickableSequentialChain
  from chainercv.utils.testing import ConstantStubLink
label_desc: Use image node_id 4 for calling the PickableSequentialChainTestBase obj's underlying member method code with example usage: obj.test_layer_names() without return types