Support for separate snapshot directory

[edmundmiller]

An example path https://github.com/jestjs/jest/blob/main/examples/snapshot/__tests__/__snapshots__/link.test.js.snap

bugs
├── snapshots
│  └── no_underscore.nf.test.snap
├── no_underscore.nf.test

Since nf-core is recommending "1 test per file" as a guideline, currently. My test directory is getting pretty cluttered.

I'll probably move to bugs/no_underscore/main.nf.test instead and house the other files needed in there.

Just wondering if there was any interest in this feature generally or not.