feat(ci): Add unit tests to CI and improve PR generation

[alissonperez]

What was done?

This PR includes several changes:

1. Added unit tests to the CI pipeline.
2. Fixed an issue with the PR generation where the llm json output parsing was not working correctly.
3. Improved the system prompt with a better description.

How was it done?

1. The unit tests were added to the CI pipeline by modifying the Github workflow files. The 'lint.yaml' file was deleted and a new 'pull_request.yaml' file was created to include both linting and testing jobs.
2. The issue with the PR generation was fixed by adding a hack to handle cases when llm returns an invalid json.
3. The system prompt was improved by modifying the 'get_pr_data' function in 'prdata.py' to provide a more detailed description of the assistant's role.

How was it tested?

1. The changes to the CI pipeline were tested by running the Github actions on the latest commit.
2. The fix to the PR generation was tested by running the 'get_pr_data' function with different inputs and verifying the output.
3. The improvement to the system prompt was tested by running the 'get_pr_data' function and verifying that the new prompt is displayed correctly.
4. A new unit test was added to 'test_prdata.py' to test the '_parse_json' function.