Improve readability of fs.test.ts; correct TS problems in it.

[weedySeaDragon]

I recently created a generator and read through quite a few of the tests to learn how to write it and how to write the tests for it.
I think that some of the tests could be made more readable and thus easier for folks to read as specifications.

I'd like to help out with this (assuming it's of value to the project) and along the way also help fix some TypeScript errors in the tests.

As a start, I've worked on fs.test.ts and will submit a PR for it.

(This work is a chore and not really a feature or a bug. I picked the 'feature' issue template since nothing is broken and thus choosing the 'bug' issue template didn't seem right at all.)