Add pytest markers for slow/integration tests

[travisjneuman]

Some test files in higher levels (6-10) could benefit from @pytest.mark.slow or @pytest.mark.integration markers. This lets learners run fast tests separately.

Scope: Review test files in levels 6-10 and add appropriate markers.

[travisjneuman]

Resolved in 36dceb1: Added pytest.mark.slow (8 tests) and pytest.mark.integration (35 tests) markers across levels 6-10. Registered markers in pyproject.toml.