Make example group alias definitions idempotent w/ no warnings.
Remove unneeded sandbox config setting.
Cleanup sandboxing.rb formatting.

There's no need for the begin/ensure because RSpec handles errors
within examples and so an example failure will not propagate up
to an around hook.
Cleanup sandbox implementation a bit.

- No need for `new_world` or `new_config` locals.
- Align assignments.
No need to name args since we ignore them all.
Clarify that this file must be required.
Add changelog for #1808.