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.