curriculum: add actionable assertion messages to testing checks

[mercael91]

Closes #95

What changed

Both files listed in the issue now report actionable messages on the 5 bare
assertions, and nothing else in the curriculum is touched:

• checks/testing/testing8.py — the three spot-check assertions name the
  expected celsius value and the returned one.
• checks/testing/testing10.py — the empty-list and negative-list assertions
  name the expected sum and the returned one.

Style

CONTRIBUTING.md asks for the actual value "where it is cheap, but never
re-call the learner's function inside the message". Each call is bound once
(celsius_32 = fahrenheit_to_celsius(32)) and the message reads that binding,
so the learner's function runs exactly as often as before. Every added line
stays inside the 80-column limit (the one 85-char line in testing10.py:5 is
unchanged from dev).

Assertion predicates, ordering, side effects and the success prints are
unchanged (testing8 ok / testing10 ok).

Verification

• AST probe from the issue: 0 bare assertions in both files (was 3 + 2).
• python -m pytest tests/integration/test_solution_verify.py -q passes: all
  reference answers verify against both checks.
• With a deliberately wrong answer the new messages fire and name the value,
  e.g. sum_list([]) should be 0, got 42 and
  fahrenheit_to_celsius(32) should be 0.0, got 999.0.
• No new line over 80 columns; the code stays Python 3.9 compatible.
• CI: all 8 matrix jobs green.

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