Enable parallel_tests for test-on-cng pipeline

What does this MR do and why?

• Fix issue with using same environment variable to trigger parallel run and detect if test is running as a parallel process which can lead to incorrect detection of parallel process.
• Update retry function to correctly merge parallel run results
• Enable parallel for test-on-cng

Closes: gitlab-org/quality/quality-engineering/team-tasks#3034 (closed)

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