Add a public testing module

[gabrieldemarmiesse]

Describe the feature and the current behavior/state.

Currently, tensorflow offers some low level functions and classes for testing: https://www.tensorflow.org/api_docs/python/tf/test

But overall, we show in the test suite that we lack tests that we could apply to any keras objects as seen in #992

My proposal is to take #992 and that we expose publicly some of the functions created. I can be a codeowner. Some good candidates would be convergence tests for optimizers, a copy of layer_test for layers and serialization tests for all keras objects.

We could then use those functions in our test suite and other projects could use them too.

Relevant information

• Are you willing to contribute it (yes/no): yes
• Are you willing to maintain it going forward? (yes/no): yes
• Is there a relevant academic paper? (if so, where): no
• Is there already an implementation in another framework? (if so, where): keras-team/keras tf.keras
• Was it part of tf.contrib? (if so, where): no

Which API type would this fall under (layer, metric, optimizer, etc.)

tfa.testing

Who will benefit with this feature?

Everyone who make new keras objects and want to test them.

Any other info.

#992

[seanpmorgan]

TensorFlow Addons is transitioning to a minimal maintenance and release mode. New features will not be added to this repository. For more information, please see our public messaging on this decision:
TensorFlow Addons Wind Down

Please consider sending feature requests / contributions to other repositories in the TF community with a similar charters to TFA:
Keras
Keras-CV
Keras-NLP