class_docstr: Test the iteration functionality provided by vcf.VcfReader.
code: def test_c_reader(self):
      self.assertNotEqual(self.sites_reader.c_reader, 0)
      self.assertNotEqual(self.samples_reader.c_reader, 0)
  
      tfrecord_reader = vcf.VcfReader(
          test_utils.genomics_core_testdata(
              "test_samples.vcf.golden.tfrecord"
          )
      )
      self.assertNotEqual(tfrecord_reader.c_reader, 0)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from absl.testing import absltest
  from absl.testing import parameterized
  from etils import epath
  from third_party.nucleus.io import vcf
  from third_party.nucleus.protos import reference_pb2
  from third_party.nucleus.protos import struct_pb2
  from third_party.nucleus.protos import variants_pb2
  from third_party.nucleus.testing import test_utils
  from third_party.nucleus.util import ranges
label_desc: Use image node_id 6 for calling the VcfReaderTests obj's underlying member method code with example usage: obj.test_c_reader() without return types