New test_gpu profile + nf-test for araregistration subworkflow

[CaroAMN]

PR checklist

The new test_gpu profile runs the full workflow on the minimal test dataset with the 3dunet on GPU.
Added nf-test for the araregistration subworkflow. Only stub is possible, since registration only works on a whole mouse brain image.

• [ x] 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/lsmquant 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).