[Task/Feature/Issue] Complete the port from `unittest` to `pytest`

[deanwampler]

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Task/Feature/Issue Details

pytest is the de facto test harness standard for Project Tapestry. We want to use it here, too, for consistency and also reuse of make tooling.

The previous PR for the refactored make process switched to pytest, but relied on its ability to run tests defined using unittest, so they currently run fine. This task is to remove the unittest dependency by porting over the code that uses it's features:

• Change classes that use unittest base classes to use pytest equivalents or convert to features.
• Switch to pytest-style assertions.
• Remove unittest from pyproject.toml.
• Confirm the test suites run successfully.

Additional Context

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