Extend test/task cancellation support to test case evaluation.

[grynspan]

This PR extends the special-casing of SkipInfo and CancellationError added in #1284 to also cover test case evaluation during test planning. If either error is thrown from evaluateTestCases(), we treat it as test cancellation the same way we do for trait evaluation.

Example:

func websites() async throws -> [Website] {
  guard let www = Web() else {
    try Test.cancel("The Web doesn't exist, is this 1992?")
  }
  return try await www.downloadEverything()
}

@Test(arguments: try await websites())
func browseWeb(at website: Website) { ... }

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