Add automated tests for nf-core download

[drpatelh]

Description of feature

As discussed on Slack, nf-core download started failing with the nf-core/rnaseq pipeline because the container definitions were non-standard and this broke the parsing code we have in tools here.

Firstly, we could add a --dry-run type option to the nf-core download command that just runs the function linked above and doesn't actually download anything. Secondly, we could add the nf-core download --dry-run <OTHER_OPTIONS> as a CI test to the pipeline template linting.yml file that checks that that the command works whenever the pipeline code is updated.

[ewels]

Not sure how clear this is, but I think just adding a lint test for correct format of containers is sufficient for most stuff.

Then adding an option to print the container strings would be a good bonus.

[ewels]

I think that --dry-run would still be quite a nice feature, so leaving this issue open for now. I just think that for testing we should use the lint tests. Keep things together.