Fix pytest vs unittest inconsistency in workshop content

[scubaninja]

Problem

Two workshop files reference pytest commands but the project uses unittest:

• content/full-day/4-testing.md (line 96): shows python -m pytest test_app.py -v
• content/github-actions/3-running-tests.md (line 96): shows pytest syntax

Fix

• Align test runner references with actual project usage
• Either update content to use unittest, or add pytest to requirements and ensure tests work with it