test(gatsby): check if worker can access node created in different process

[pieh]

Description

Building on #31768

This adds setup and helpers to be able to execute tests covering workerpool. This adds single test that checks if worker can get node created by different process. But main point of this PR is to setup test helpers and structure for the future.

Overview of setup:

• createTestWorker creates actual worker (separate process)
• packages/gatsby/src/utils/worker/__tests__/test-helpers/child-for-tests.ts is a place to add additional helper functions so tests could interact with worker that shouldn't be available in actual production worker pool
• itWhenLMDB is just convience wrapper around jest's it to only execute tests when LMDB_STORE flag/env is set

Related Issues

[ch32034]