docs(CONTRIBUTING): add tests section

[svaloumas]

Description

This PR adds a minimal Tests section in the CONTRIBUTING.md.

Motivation and Context

We need to document somewhere the process of uniformly generating mocks and the CONTRIBUTING.md might be a good place to do so. In addition, this section could be used to add more about our test conventions/guidelines or any other related information in the future.

How Has This Been Tested?

Screenshots / Output (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Base: 63.55% // Head: 63.55% // No change to project coverage 👍

Coverage data is based on head (e709c49) compared to base (0c3d986).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #392   +/-   ##
=======================================
  Coverage   63.55%   63.55%           
=======================================
  Files          30       30           
  Lines        1029     1029           
=======================================
  Hits          654      654           
  Misses        316      316           
  Partials       59       59           

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