tests: improve TAP skip reason reporting with specific explanations

tests: improve TAP skip reason reporting with specific explanations

Enhance the TAP test framework to support and display specific skip reasons:
- Modify run_all_tests() to handle (return_code, reason) tuples
- Update tests to return (77, reason) instead of just 77
- Add descriptive skip reasons like "requires root privileges"
- Show skip reasons in TAP output as "#SKIP reason"

This makes test output more informative by explaining why tests
were skipped rather than showing generic skip messages.

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Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>