Refactor tests

[Aratz]

This is the second attempt to make it easier and faster to use the tests when developing the pipeline (first attempt was PR #161)

This PR does the following:

• Merge all mpx pipeline tests, and all pna pipeline tests
• Execute the experiment summary in test mode
• Add missing stub tests for PNA modules
• Snapshot only the file list and the version file, for all tests. Content validation is done internally in the pixelator tests
• Add script to run the tests in parallel locally
• Tag one test as smoke test for quick diagnostics and add a script to run it.

PR checklist

• This comment contains a description of changes (with reason).
• If necessary, also make a PR on the nf-core/pixelator 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>).
• CHANGELOG.md is updated.

[fbdtemme]

Very nice!
Only thing I am unsure about is the test scripts. I have not really seen any custom scripts being used in other nf-core repos so maybe the core folks would have issues with that?

I would just leave those out.

[johandahlberg]

Maybe it is something to bring to the table and hear what the nf-core folks think about it? Since testing seems to be a pain point for main pipeline as I understand it.

[Aratz]

I can ask on slack and fix it in a new PR if I get any suggestions, thanks for the review!