class_docstr: Test the functionality provided by vcf.InMemoryVcfReader.
code: def setUp(self):
      self.variants = [
          test_utils.make_variant(chrom="1", start=10),
          test_utils.make_variant(chrom="1", start=20),
          test_utils.make_variant(chrom="1", start=30),
          test_utils.make_variant(chrom="2", start=25),
          test_utils.make_variant(chrom="2", start=55),
          test_utils.make_variant(chrom="3", start=10),
      ]
      self.header = variants_pb2.VcfHeader(
          contigs=[
              reference_pb2.ContigInfo(name="1", n_bases=100),
              reference_pb2.ContigInfo(name="2", n_bases=100),
              reference_pb2.ContigInfo(name="3", n_bases=100),
              reference_pb2.ContigInfo(name="4", n_bases=100),
          ],
          filters=[],
          sample_names=["NA12878"],
      )
      self.reader = vcf.InMemoryVcfReader(self.variants, self.header)
  
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 1 for calling the InMemoryVcfReaderTests obj's underlying member method code with example usage: obj.setUp() without return types