Cleanup dry_run cuke.

- Favor curly-brace blocks over single-line do/end with semicolons.
- No need to use a background give it's just one scenario.
- Put the spec in the normal spec directory.
Improve dry-run description.
Don't use `or` in conditionals.

It's a control flow operator, not a conditional operator.
Prefer predicate form of boolean config option.
Simplify spec.

- It is simpler and less maintenance to collapse it into one.
- The sandboxing of config is already handled for us.
- It handles `around` hooks, too.
Dry run should skip :suite hooks, too.

...since it is advertised as skipping hooks.
Changelog entry for --dry-run.
Merge pull request #1056 from rspec/dry_run_improvements

Dry run improvements