[movie-lister] Added test fixture and updated documentation

[mrKazzila]

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Link to the current version: tutorials/cli

Changes Made

This pull request proposes the following changes:

• Added a new test fixture finder_mock for examples/miniapps/movie-lister/movies/tests.py.
• Updated the example in the documentation to reflect the use of the new fixture.

Why

These changes allow for:

• Reduced code duplication.
• Updating the documentation makes it more relevant and informative for framework users.

Testing

Didn't notice any problems, just run pytest movies/tests.py --cov=movies and pytest movies/tests.py from python-dependency-injector/examples/miniapps/movie-lister directory.