Add a `Bundle.testTarget` property.

[grynspan]

This PR adds an experimental class property to Bundle in the Foundation cross-import overlay. The property's value represents the bundle containing the test target. On Apple platforms, this is an XCTest bundle. Elsewhere, it's just the main bundle.

This is an experimental interface only.

Checklist:

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

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test Windows

[grynspan]

The existence of #bundle supersedes this PR. https://github.com/swiftlang/swift-foundation/pull/1274