Fix `ResourceWarning: unclosed file` warnings in reader_ops_test (#3827)

Some file handles created in these test cases are were not being closed.
This causes some warnings as well along with leaking the handles.

./source/tensorflow/tensorflow/python/kernel_tests/reader_ops_test.py:245: ResourceWarning: unclosed file <_io.BufferedWriter name='/var/folders/sq/vmncyd7506q_ch43llrwr8sn6zfknl/T/reader_ops_test/text_line.0.txt'>
  f = open(fn, "wb")
/source/tensorflow/tensorflow/python/kernel_tests/reader_ops_test.py:276: ResourceWarning: unclosed file <_io.BufferedWriter name='/var/folders/sq/vmncyd7506q_ch43llrwr8sn6zfknl/T/reader_ops_test/text_line.1.txt'>
  self._testOneEpoch(self._CreateFiles(crlf=True))
./source/tensorflow/tensorflow/python/kernel_tests/reader_ops_test.py:273: ResourceWarning: unclosed file <_io.BufferedWriter name='/var/folders/sq/vmncyd7506q_ch43llrwr8sn6zfknl/T/reader_ops_test/text_line.1.txt'>
  self._testOneEpoch(self._CreateFiles(crlf=False))
./source/tensorflow/tensorflow/python/kernel_tests/reader_ops_test.py:279: ResourceWarning: unclosed file <_io.BufferedWriter name='/var/folders/sq/vmncyd7506q_ch43llrwr8sn6zfknl/T/reader_ops_test/text_line.1.txt'>
  files = self._CreateFiles()
../source/tensorflow/tensorflow/python/kernel_tests/reader_ops_test.py:183: ResourceWarning: unclosed file <_io.BufferedWriter name='/var/folders/sq/vmncyd7506q_ch43llrwr8sn6zfknl/T/reader_ops_test/whole_file.0.txt'>
  open(fn, "wb").write(c)
/source/tensorflow/tensorflow/python/kernel_tests/reader_ops_test.py:183: ResourceWarning: unclosed file <_io.BufferedWriter name='/var/folders/sq/vmncyd7506q_ch43llrwr8sn6zfknl/T/reader_ops_test/whole_file.1.txt'>
  open(fn, "wb").write(c)
/source/tensorflow/tensorflow/python/kernel_tests/reader_ops_test.py:183: ResourceWarning: unclosed file <_io.BufferedWriter name='/var/folders/sq/vmncyd7506q_ch43llrwr8sn6zfknl/T/reader_ops_test/whole_file.2.txt'>
  open(fn, "wb").write(c)
...