rngtest, run-test.sh: Robust error handling, dependency SKIP, and summary improvements

[smuppand]

This PR enhances the Kernel rngtest test script and the main run-test.sh runner with several reliability, logging, and error-handling improvements:

• rngtest/run.sh:

  • Uses dd instead of cat to reliably generate input entropy, preventing hangs or input depletion from /dev/random.
  • Adds stricter dependency checks (for both rngtest and dd) and SKIP logic if binaries are missing.
  • Provides clear error handling and logs if reading, executing, or parsing results fails.
  • Cleans up all temporary files after execution, whether PASS or FAIL.
  • Explicitly validates and logs the parsed bit error count, failing gracefully if output is invalid.

• run-test.sh:

  • Correctly recognizes and propagates SKIP status from test scripts (e.g., when dependencies are missing).
  • Does not treat SKIP as PASS in summaries.
  • Prints clear test summary sections for PASSED, FAILED, and SKIPPED tests.
  • Improves modularity and logging to assist with CI pipeline diagnostics.

Together, these changes ensure test results reflect true execution status and failures, reduce flakiness in CI, and allow for more consistent debugging and summary reporting.

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Tested on multiple setups with and without missing dependencies, and confirmed proper SKIP and summary behavior.

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