chore: allow opt-out out of slow tests

[sander2]

Allow opting out out of slow tests - this greatly speeds up tests locally. I chose to make it opt-out instead of opt-int so we can be sure that the CI includes everything.

I suggest setting up your vs code to use cargo test --workspace --features skip-slow-tests --lib --bins --tests. This skips 3 slow tests and skips doc-tests (which we do not use, but are slow to run even so). This runs in 13 seconds, compared to the 2m25s that cargo test --workspace takes.