convert pytests to nf-test for concatenation

[maxulysse]

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• If necessary, also make a PR on the nf-core/sarek branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).

[nf-core-bot]

Warning

Newer version of the nf-core template is available.

Your pipeline is using an old version of the nf-core template: 3.0.2.
Please update your pipeline to the latest version.

For more documentation on how to update your pipeline, please see the nf-core documentation and Synchronisation documentation.

[erikrikarddaniel]

👍
I thought we were supposed to have pipeline tests in tests/pipeline. Let's see what you'll say in the forthcoming documentation! ;-)

[erikrikarddaniel]

Trailing whitespace?

[maxulysse]

that's generated by nf-test, It's because I'm snapshoting an empty list.
I'll see about that in subsequent PRs.
Thanks @erikrikarddaniel