Disable test suites that contain no tests

I haven't looked closely at this, but I'm hopeful that some compile time can be saved by not building empty test suites and doc test suites.

I think cargo has some ability to control which targets are built for testing. I hope it can control which are doc-tested.

So the task here is to find all of the empty test suites run by cargo test --all and disable them in Cargo.toml by setting either test = false or doctest = false. Then time cargo test --all --no-run before and after.