Provide integrations with Swift Testing exit tests

(Will add more detail when I return from leave.)

Swift Testing is introducing a new "exit tests" feature in Swift 6.2. Would be great if we could find a way to integrate it with the argument parser, e.g. something like:

```swift
await #expect(arguments: ["--foo", "bar", ...], exitWith: .success)
```

Start by reviewing Swift 6.2 Swift Testing exit tests and the argument parser's existing testing integration points. Define how an argument invocation should be checked against an expected exit result, using the proposed await #expect(arguments:exitWith:) example as the target. Done means the integration behavior and API are specified and covered by tests.

Written by the indexing model from the issue text.