feature: add tests

[ariadnafer]

Description

Added testing with coverage
Added tests for the following components:

• extract_bq_to_dataset
• custom_train_job
• import_model_evaluation

How has this been tested?

This PR is for testing purposes itself

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have successfully run the E2E tests, and have included the links to the pipeline runs below
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated any relevant documentation to reflect my changes
• I have assigned a reviewer and messaged them

Pipeline run links:

[felix-datatonic]

Thanks @ariadnafer! Have left a minor comment on formatting mock.patch, otherwise happy with improving the code coverage. Also, I'd remove inline comments as much as possible and only comment complex code blocks. Python code should aim for simplicity and self-explainability.

[ariadnafer]

/gcbrun