refactor: unnest and simplify oohelper implementation

[bassosimone]

While working on https://github.com/ooni/probe-cli/pull/805, I noticed that oohelper's implementation could really use a cleanup and refactoring. Here's what we should roughly do:

• [ ] unnest the implementation (long time ago, I was creating many small interconnected packages while now I have settled on creating a large, flat set of packages)
• [ ] make sure we're using unit tests rather than integration tests
• [ ] we should use mocks from internal/model/mocks rather than rolling out our own mocks
• [ ] if we need integration tests, we should be testing against the oohelperd implementation in the same codebase

This refactoring is not very important because the existing code is working ~fine, but would improve the state of the tree.

[bassosimone]

Turns out we already did this but forgot to close the issue!

[bassosimone]

My mistake, this is about oohelper and not oohelperd 😢