Improve rngtest CI reliability with output-based validation and success ratio logging

[smuppand]

This update improves the robustness and CI compatibility of the rngtest functional test by:

• Removing dependency on the rngtest return code, which may be unreliable even for acceptable success ratios.
• Parsing and validating based on FIPS 140-2 success count extracted from output.
• Allowing minor failure tolerance by accepting tests with ≥ 997 out of 1000 successes.
• Logging exact success/failure ratio for better visibility, even on failure.
• Cleaning up unused variable (RNGTEST_RC) to resolve ShellCheck SC2034 warning.
• Preserving overall format and logging style in line with qcom-linux-testkit test structure.

[vnarapar]

LGTM