consider test-splitting strategy for expensive e2e tests

[edwardfoyle]

Description

As our e2e suite grows it will become more important to scale e2e runs horizontally. When / if the need arises, we should consider a test-splitting strategy to run many jobs in parallel.

Currently we use matrix actions and my opinion is that we should pursue this method of parallelization as far as it will take us. After that we could look into more creative options like generating GH workflow files.