Update test readme

[pradnyeshjoshi]

Description

• Added details on how to add a new test to the AzureML pipeline to the test readme.
• Reorganized test readme.

Related Issues

Checklist:

• I have followed the contribution guidelines and code style for this project.
• I have added tests covering my contributions.
• I have updated the documentation accordingly.
• This PR is being made to staging branch and not to main branch.

[miguelgfierro]

Have just done this PR #1777 to your branch.

One minor thing I noticed. In the file https://github.com/microsoft/recommenders/blob/miguel/test_doc/tests/ci/azureml_tests/test_groups.py we use the word spark in the nightly builds and pyspark in the unit tests, can we choose the same for both?

[pradnyeshjoshi]

Have just done this PR #1777 to your branch.

One minor thing I noticed. In the file https://github.com/microsoft/recommenders/blob/miguel/test_doc/tests/ci/azureml_tests/test_groups.py we use the word spark in the nightly builds and pyspark in the unit tests, can we choose the same for both?

I merged PR #1777 into my branch and changed pyspark to spark.

[miguelgfierro]

This is good, I have done a typo review, feel free to review and change if you see more typos.