test(core): parallelise core test suite

[bernardobridge]

Description

Lengthy PR because there was a lot of reshuffling/refactoring that was needed for some of this. Reviewing commit by commit might be easier.

Reduces test runtime for core from 9 minutes to <2 minutes locally. Overall it reduces test runtime in CI to about 4-5 minutes. A few things:

• Some follow up PR's will be added to just refactor some of the naming conventions to grep easier, but didn't want to make this PR too messy. Also there are a couple of things I want to clean up
• A couple of tests were quarantine'd right now, as I need to review why they're not working separately
• Some further optimisation is still possible, I'll explore it separately
• Funny enough, now the longest runtime thing is our examples - using-data-from-csv is an unnecessarily long test (8 mins), maybe we should just reduce that too