[DRAFT] Add simple hello-nf-test walkthrough

[adamrtalbot]

This update adds the introduction to nf-test:

The idea:

• Have a simple nf-test tutorial which uses hello world. Users can follow it and get a very easy introduction to nf-test (~1/2 day)
• Have a more complicated example with hello-genomics which includes more advanced concepts of nf-test (~1/2 day)
  Making a full day of nf-test training.

So here we introduce nf-tests to the hello-world pipeline, keeping it as simple as possible given the context.

It removes the file moving and restructuring that happens in hello-genomics and focuses on the why and how of testing.

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[pinin4fjords]

closing in favour of #516 and a future separate PR for the genomics component