Change default behavior of exit tests to always succeed.

[grynspan]

Exit tests simulate calling main() (or some similar thing), but if you don't explicitly exit from them, they force a failure. After much mulling and some discussion with colleagues a while back, we should change the behavior so that, if a test doesn't otherwise terminate, it acts as if main() returned naturally—that is, by exiting with EXIT_SUCCESS rather than by forcing EXIT_FAILURE. This behavior is more consistent with the feature's theory of operation.

For example:

await #expect(exitsWith: .success) {
  assert(2 > 1) // this is true and doesn't assert, so this test should pass, right?
}

Exit tests remain an experimental feature.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test