Replace deprecated test_session with cached_session

While running tests:
```
bazel test -s --verbose_failures --config=opt //tensorflow/python:image_ops_test
```

there are many deprected warnings:
```
From /usr/lib/python2.7/contextlib.py:84: test_session (from tensorflow.python.framework.test_util) is deprecated and will be removed in a future version.
Instructions for updating:
Use `self.session()` or `self.cached_session()` instead.
```

This fix replace deprecated test_session with cached_session.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>