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.
This causes problems when used in conjunction with `#guard_msgs` (which
checks whitespace) and trailing whitespace removal. Discovered by
@PatrickMassot in verbose-lean4.
