Implement tests with pytest-workflow

[grst]

Pytest workflow makes it easier to maintain larger test suites and also allows to check outputs.

cf. https://github.com/nf-core/tools/issues/605, https://github.com/nf-core/rnaseq/pull/546, https://github.com/nf-core/sarek/tree/dev/tests

Testing scenarios:

• Run pipeline with more than one file
• different parameter combinations
• different filenames and folder structures for cellranger
• run subworkflows with and without index
• different protocols, if we manage to get the test files