rngtest: Improve robustness and ensure correct CI exit codes

[smuppand]

This PR improves the rngtest validation script for CI use:

• Uses dd (not cat) to reliably collect entropy for rngtest input.
• Adds explicit exit 0/1 codes at every branch to ensure CI reliably detects PASS/FAIL.
• Handles errors from all random data steps, including drained entropy and missing output.
• Always cleans up temporary files, regardless of outcome.
• Adds better debug logs and clarifies error messages for failed tests.
• Output file naming and parsing is clarified for maintainability.

This ensures CI or manual runs always have robust, predictable results.

[vnarapar]

LGTM