class_docstr: Test the iteration functionality provided by vcf.VcfReader.
code: def test_vcf_iter(self):
      n = 0
      for _ in self.sites_reader:
          n += 1
      self.assertEqual(n, 5)
  
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 4 for calling the VcfReaderTests obj's underlying member method code with example usage: obj.test_vcf_iter() without return types