This PR promotes issue handling traits (namely, the `IssueHandlingTrait` type) to non-`@_spi` public API, now that it has been accepted by the Testing Workgroup.

[heeb0o-d]

This PR promotes issue handling traits (namely, the IssueHandlingTrait type) to non-@_spi public API, now that it has been accepted by the Testing Workgroup.

Checklist:

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

Originally posted by @stmontgomery in swiftlang/swift-testing#1198

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