Add nf-test for md5sum module

[human9]

Checks that the correct filename and contents are created. A custom config file is used to test extra args.

PR checklist

Closes #5159

• [X] nf-test test modules/nf-core/md5sum/tests/main.nf.test --profile docker

[human9]

Hmm, I don't understand why the conda test failed...

[SPPearce]

Hmm, I don't understand why the conda test failed...

The conda test was failing because conda and docker/singularity were using different versions. I've downgraded the conda version to match the ubuntu base (by copying what was used in another module).