Implement some kind of validation tests

[ewels]

It would be nice to add in some checks for pipeline output files when -profile test is used. This will need to be specific for all pipelines, but we could add an example here.

I think the simplest would be to have a bunch of processes which are downstream of the normal ones and have when: workflow.process.contains('test') set so that they don't run normally.

See extensive discussion on nextflow gitter from here.

[ewels]

This issue was moved to nf-core/tools#94