Use JSON test mapping in `detect-tests` job

Use JSON test mapping in detect-tests job

part of #222369 (closed):

  1. In MR pipelines, fetch test mapping from S3, and feed it to test_file_finder to list test files that needs to be executed.

This would require https://gitlab.com/gitlab-org/ci-cd/test_file_finder/-/issues/15 to be implemented first.

Implemented in !49091 (merged)

Edited by Albert Salim