Correctly handle explicitly passed specs when running tests in parallel
What does this MR do and why?
Fix long standing issue where explicitly passing specs to bin/qa script would not work correctly with parallel runners, specifically knapsack because it would always be added to the list of tests returned by knapsack test allocator.
This fix also allows to simplify dynamic pipeline generator because it removes the needed workaround to manually set up knapsack pattern on pipeline job level instead of QA_TESTS variable.
This mr is also a precondition to [E2E] Improve knapsack parallel job split for E... (gitlab-org/quality/analytics/team#28 - closed) that should allow to create test split that would resemble exactly the tests that would get executed.
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