implement sharding of tests

[maxulysse]

Adapted from nf-core/sarek#1668 by @edmundmiller | @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/rnaseq 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).

[edmundmiller]

Awesome! I can't take too much credit. It was adapted from https://www.nf-test.com/tutorials/github-actions/

[maxulysse]

Let's put it aside for now, I think it's the way to go, but let's try to see if #1437 is not fixing our issues already