Set QA_TESTS from code paths mapping

What does this MR do and why?

Relates to gitlab-org/quality/quality-engineering/team-tasks#2396 (closed)

Note that this MR only takes care of setting QA_TESTS at detect_changes stage. The actual CI job changes will be part of a subsequent MR in the spirit of iteration 😃

Testing

• Code changes (with SELECTIVE_EXECUTION set to true) - should run only selective tests ie., QA_TESTS should be set from mapping file
• Code changes (with SELECTIVE_EXECUTION set to false) - should run selective tests the old way using stage label,. QA_TESTS should be set from stage labels
• E2E spec changes - Run only changed specs (current behaviour continues) - QA_TESTS should be set to only changed E2E specs
• QA framework changes - Runs full suite - QA_TESTS should not be set

Scenario	Test MR and job
Code changes (with SELECTIVE_EXECUTION set to true)	QA_TESTS is set for this test MR
Code changes (with SELECTIVE_EXECUTION set to false)	QA_TESTS is not set and with stage label set, QA_TESTS is set
E2E spec changes (with SELECTIVE_EXECUTION set to true)	QA_TESTS is set for Test MR
QA framework changes (with SELECTIVE_EXECUTION set to true)	QA_TESTS is not set for test MR

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Before	After

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