run-tests.sh - separate test allocation to bins into a TestWorkAllocator class

Problem/Motivation

run-tests.sh has logic to split the list of tests to be executed to separate bins that can be run in parallel to leverage GitLabCI parallelization capabilities.

This logic is intertwined with the test discovery process and cannot be tested ATM.

Proposed resolution

1. separate test allocation to bins into a TestWorkAllocator class, taking as input the output of the test discovery
2. add unit testing for the new class

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet