Implement nf-test to test modules

[gregorysprenger]

Description of feature

Current behavior:
Workflow only has end-to-end testing to test if the pipeline starts and finishes via nextflow run main.nf -profile singularity,test --outdir results.

Expected behavior:
Implement tests for each module, possibly through the use of nf-test. If all else fails, use Python's pytest or UnitTest?