feat(cli): expose dirname utility to test scripts

[bernardobridge]

Description

This will be useful when it's necessary to reference a path in the test config, but it's not one we know to resolve automatically (e.g. processors and payloads).

One known use case right now is the Playwright Engine's storageState. Our current example to reuse authentication by leveraging storageState is broken, and this is one way to fix it.

Note: I first cleaned up existing logic a little bit (9da818a), so best to review commit by commit.

Testing

I intend to add an automated test, but didn't have time yet. I've tested manually against Fargate and locally (using the example), and it works.