Implement nf-test

[WackerO]

Description of feature

I'm going to switch the github ci to pytest-workflow so that github can monitor the consistency of pipeline output instead of just checking that PRs don't make the pipeline fail

[apeltzer]

Rather go for nf-test please, not pytest anymore.

[WackerO]

I will move this back a bit until the hackathon, maybe nf-test can be discussed then (if this is the recommended nf-core/nextflow testing tool, it would be great to also have a bit more documentation, e.g. how to best test multiple profiles of a pipeline)