As a person who once struggled to parallelize a test suite when the entire thing was not written with parallelization in mind (until its runtime became onerous)... I would highly, highly recommend using some parallelization framework from the get-go to uncover any concurrency issues immediately.

In Elixir this is literally as simple as adding "async: true" to your test modules.

It will also force you to write tests that can run concurrently... which has a few nice side effects for your code design generally.