Add tests to make sure python modules are exposed correctly

[yongtang]

While tensorflow-io has tests covered for different modules (Ignite/Kafka/etc) through bazel test, those tests are always run inside the repo directory and no through pip install.

It would be good to have tests to make sure pip install correctly exposes python modules. Some simple tests of pip install tensorflow-io-*.whl && python -c "import tensorflow_io.Kafka" would be good enough to serve the needs here.

[juwangvsu]

run test inside the repo is fine. It would be nice to have a section in the README about test procedure and
location of the test code.

[yongtang]

Each op may have different test procedures. For example, Kafka requires setting up a docker container of kafka before running test. I think individual README.md at each repo would be nice.

[yongtang]

An integration test has been added to tensorflow/io/tests. The integration test is invoked through pip install tensorflow-io-*.whl && python -m pytest . so the exposure of python modules could be validated. It covers video format only at the moment but could be a start.

[yongtang]

Think this issue is resolved with #118.