test: Group up Pipeline unit tests in a single class

[silvanocerza]

Proposed Changes:

Group up Pipeline unit tests in test_pipeline.py file.

Those are not all the unit tests there are, following PRs will bring other unit tests from other files too.

This is done in preparation of changes that will make easier to test Pipeline.

How did you test it?

I ran tests locally.

Notes for the reviewer

N/A

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[coveralls]

Pull Request Test Coverage Report for Build 9118137499

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.587%

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Totals
Change from base Build 9116506765:	0.0%
Covered Lines:	6592
Relevant Lines:	7277

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💛 - Coveralls

[masci]

Thank you!

[silvanocerza]

Part of #7611