fix(rpc): modify shutdown used in `stop()`

[oxarbitrage]

Motivation

The std::process::exit I introduced for the stop RPC method is not a good idea for the rpc testing because (among other issues) it terminates immediately and the tests require for the method to return before actually finishing with the current process.

Additionally this is a more graceful shutdown and will close https://github.com/ZcashFoundation/zebra/issues/8850

Solution

Terminate the process by sending a SIGINT signal.

PR Author's Checklist

• [x] The PR name will make sense to users.
• [ ] The PR provides a CHANGELOG summary.
• [ ] The solution is tested.
• [ ] The documentation is up to date.
• [x] The PR has a priority label.

PR Reviewer's Checklist

• [ ] The PR Author's checklist is complete.
• [ ] The PR resolves the issue.