This PR migrates most remaining tests to the new test suite. It also
completes the migration of directories like `tests/lean/run`, meaning
that PRs trying to add tests to those old directories will now fail.
This PR sets up the new integrated test/bench suite. It then migrates
all benchmarks and some related tests to the new suite. There's also
some documentation and some linting.
For now, a lot of the old tests are left alone so this PR doesn't become
even larger than it already is. Eventually, all tests should be migrated
to the new suite though so there isn't a confusing mix of two systems.
