Restore single class MatthewsCorrelationCoefficient test cases

[lgeiger]

Description

This PR restores single class tests for MatthewsCorrelationCoefficient. These tests have been removed in #2406 which unfortunately broke MatthewsCorrelationCoefficient with num_classes=1. This was a breaking change which doesn't seem like it was intended. These test will currently fail on master but passed in 0.12.1 (or any version before #2406).

Please consider this PR as an issue report. Unfortunately I don't have time time right now to properly look into fixing this issue, but maybe @jonpsy knows more about this. fixed in #2495

/cc @jonpsy @bhack @seanpmorgan

Type of change

• Bug fix
• New Tutorial
• Updated or additional documentation
• Additional Testing
• New Activation and the changes conform to the activation contribution guidelines
• New Callback and the changes conform to the callback contribution guidelines
• New Image addition and the changes conform to the image op contribution guidelines
• New Layer and the changes conform to the layer contribution guidelines
• New Loss and the changes conform to the loss contribution guidelines
• New Metric and the changes conform to the metric contribution guidelines
• New Optimizer and the changes conform to the optimizer contribution guidelines
• New RNN Cell and the changes conform to the rnn contribution guidelines
• New Seq2seq addition and the changes conform to the seq2seq contribution guidelines
• New Text addition and the changes conform to the text op contribution guidelines

Checklist:

• I've properly formatted my code according to the guidelines
  • By running Black + Flake8
  • By running pre-commit hooks
• This PR addresses an already submitted issue for TensorFlow Addons
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• This PR contains modifications to C++ custom-ops

[bot-of-gabrieldemarmiesse]

@autoih @marload

You are owners of some files modified in this pull request.
Would you kindly review the changes whenever you have the time to?
Thank you very much.