Add example workflow with nf-test to nextflow CI jobs

[mattheww95]

While using nf-test with the nextflow CI jobs may create some issues due to reliance on an external piece of software for regular unit testing.

I think the addition of a small example workflow to the existing CI jobs would prevent regressions where unexpected errors occur and are only noticed after users have already updated nextflow.

I know edge releases are not expected to always work immediately but it is a real headache when your CI jobs stop working and you cant figure out why 😖