implement sharding of tests

[maxulysse]

Adapted from https://github.com/nf-core/sarek/pull/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).

[maxulysse]

Let's put it aside for now, I think it's the way to go, but let's try to see if https://github.com/nf-core/rnaseq/pull/1437 is not fixing our issues already