test: Refactor some retriever tests into unit tests

[sjrl]

Related Issues

• fixes N/A

Proposed Changes:

Refactored some retriever tests into unit tests.

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 5518230485

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 169 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.4%) to 45.234%

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Files with Coverage Reduction	New Missed Lines	%
nodes/retriever/dense.py	169	26.45%

Totals
Change from base Build 5505967772:	0.4%
Covered Lines:	10431
Relevant Lines:	23060

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💛 - Coveralls

[anakin87]

Hey @sjrl...
Thanks for adapting these tests!

I left a comment but nothing blocking...