Enable Test.cancel() as API.

Pending approval of [ST-0016](https://github.com/swiftlang/swift-evolution/blob/main/proposals/testing/0016-test-cancellation.md).

Author: grynspan

Sources/Testing/Test+Cancellation.swift

@@ -169,7 +169,8 @@ extension Test: TestCancellable {
   ///     attribute the cancellation.
   ///
   /// - Throws: An error indicating that the current test or test case has been
-  ///   cancelled.
+  ///   cancelled. The testing library does not treat this error as a test
+  ///   failure.
   ///
   /// The testing library runs each test and each test case in its own task.
   /// When you call this function, the testing library cancels the task
@@ -206,7 +207,6 @@ extension Test: TestCancellable {
   /// - Important: If the current task is not associated with a test (for
   ///   example, because it was created with [`Task.detached(name:priority:operation:)`](https://developer.apple.com/documentation/swift/task/detached(name:priority:operation:)-795w1))
   ///   this function records an issue and cancels the current task.
-  @_spi(Experimental)
   public static func cancel(_ comment: Comment? = nil, sourceLocation: SourceLocation = #_sourceLocation) throws -> Never {
     let skipInfo = SkipInfo(comment: comment, sourceContext: SourceContext(backtrace: nil, sourceLocation: sourceLocation))
     try Self.cancel(with: skipInfo)

Sources/Testing/Testing.docc/EnablingAndDisabling.md

@@ -121,19 +121,18 @@ func allIngredientsAvailable(for food: Food) -> Bool { ... }
 func makeSundae() async throws { ... }
 ```
 
-<!--
 ### End a test after it has already started
 
-TODO: document Test.cancel() and Test.Case.cancel() here
-
 If a test has already started running and you determine it cannot complete and
 should end early without failing, use ``Test/cancel(_:sourceLocation:)`` to
 cancel the test:
 
 ```swift
-...
+@Test("Can make sundaes")
+func makeSundae() throws {
+  guard let iceCreamMaker = IceCreamMaker() else {
+    try Test.cancel("The ice cream maker isn't working right now")
+  }
+  ...
+}
 ```
-
-If the test is parameterized and you only want to cancel the current test case
-rather than the entire test, use ``Test/Case/cancel(_:sourceLocation:)``.
--->

Sources/Testing/Testing.docc/MigratingFromXCTest.md

@@ -556,11 +556,8 @@ test function with an instance of this trait type to control whether it runs:
   }
 }
 
-<!-- TODO: document Test.cancel() and Test.Case.cancel() here, and update
-     relevant links to use proper DocC symbol references.
-
 If a test has already started running and you determine it cannot complete and
-should end early without failing, use `Test/cancel(_:sourceLocation:)` instead
+should end early without failing, use ``Test/cancel(_:sourceLocation:)`` instead
 of [`XCTSkip`](https://developer.apple.com/documentation/xctest/xctskip) to
 cancel the task associated with the current test:
 
@@ -593,10 +590,6 @@ cancel the task associated with the current test:
   }
 }
 
-If the test is parameterized and you only want to cancel the current test case
-rather than the entire test, use `Test/Case/cancel(_:sourceLocation:)`.
--->
-
 ### Annotate known issues
 
 A test may have a known issue that sometimes or always prevents it from passing.