Introduce test load balancing for MacOS and Windows build pipeline jobs

[emmacasolin]

Specification

Test load balancing was implemented in Typescript Demo Lib using Jest's --shard option in order to parallelise the tests there. We can do a similar thing here in order to speed up our tests. Due to upstream issues we are currently unable to include stdout/stderr output in our Junit reports on Gitlab (MatrixAI/TypeScript-Demo-Lib#66) so sharding is not suitable for our linux tests where we may need to be debugging on the CI/CD. As such, we can leave the existing child pipeline setup for our linux tests and introduce test load balancing only to our tests running on other platforms.

Additional context

• Test load balancing using jest's --shard option TypeScript-Demo-Lib#65 - PR for test load balancing on TS Demo Lib
• Apply bin packing algorithms to the setup of our parallel CI/CD test runners #393 - Issue for improving the efficiency of parallel jobs for test load balancing

Tasks

1. Port changes from TS Demo Lib to here