Fix test generation

[alice-i-cecile]

Objective

• the gentest script which automatically generates tests comparing our results to Chrome's is broken and does not compile
  • (Eventually) fixes Fix cargo run --package gentest #65.

Context

This failure was discovered in #46.

Feedback wanted

1. Do we want to keep this strategy? Or should it be replaced with careful unit tests?
2. Chrome's implementation of the spec is only one of several possible options; see Question: why layout values are rounded? #77. Its behavior isn't always desirable.
3. This strategy won't work for other layout algorithms; see Support multiple layout algorithms #28.
4. The generated code is massive, and makes refactoring remarkably painful. See Rename sprawl::node::Stretch -> sprawl::node::Sprawl #79.
5. Any help with tokio would be appreciated; I've never touched this section of Rust before.

[TimJentzsch]

Do we want to keep this strategy? Or should it be replaced with careful unit tests?

IMO it shouldn't be the main testing strategy, it's definitely a painful to set up (you need to install external tools) so we can't expect every contributor to do it. I think the main tests should be done with unit tests and we can keep something like this in CI to catch edge-cases if feasible.

[alice-i-cecile]

Closing in favor of #133. These generated tests are a disaster to maintain.