Add ability to run e2e tests in parallel within a single CI job

What does this MR do and why?

Introduces the implementation of parallel testing on test-on-omnibus pipeline using the parallel_tests gem. The aim of parallelization using multiple browsers running a subgroup of tests per CPU core is to reduce the test feedback time for the pipeline. More context on the Improve parallel test execution in E2E tests epic. This should be merged with QA_PARALLEL_RUN: false for the time being. The reason to do so is to further iterate on it, to avoid big MR reviews, so it can be switched on making sure that:

• Adjust junit report logic for parallel tests is merged
• The retry rate is around 4-7 failures per pipeline - !166287 (closed), !166288 (closed)

MR acceptance checklist

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