Introduce issue handling trait (as SPI)

[stmontgomery]

This introduces a new trait type named IssueHandlingTrait as SPI. It allows observing, transforming, or filtering the issue(s) recorded during the test it's applied to. Here's a contrived example:

@Test(.transformIssues { issue in
  var issue = issue
  issue.comments.append("A comparison of two literals")
  return issue  // Or, return `nil` to suppress the issue
})
func example() {
  #expect(1 == 2)
}

Motivation:

Sometimes it can be useful to customize an issue recorded during a test. For example, you might wish to add supplemental information to it, such by adding comments. Another example of this could be adding an attachment to an issue, which was a capability mentioned as a future direction in ST-0009 Attachments.

Other times, you might wish to suppress an issue which is later determined to be irrelevant or cannot be marked as a known issue. Or, you may simply want to be notified that an issue was recorded, to react to it in some other way while still recording the issue normally.

Checklist:

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

Resolves rdar://140144041

[stmontgomery]

@swift-ci please test

[stmontgomery]

@swift-ci please test

[stmontgomery]

The macOS CI failure is expected to be resolved by swiftlang/swift#80830, but the release of a new main snapshot toolchain is currently blocked for unrelated reasons. Proceeding to merge.