Work around test discovery crash when running on older OS versions due to unguarded use of .timeLimit (#409)

Fix a test discovery crash when running on older Apple OS versions which
don't support `Clock` and related APIs.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

Diff for: Tests/TestingTests/Test.SnapshotTests.swift

@@ -97,13 +97,33 @@ struct Test_SnapshotTests {
   private static let bug: Bug = Bug.bug(12345, "Lorem ipsum")
 
   @available(_clockAPI, *)
-  @Test("timeLimit property", .timeLimit(.minutes(999_999_999)))
+  @Test("timeLimit property", _timeLimitIfAvailable(minutes: 999_999_999))
   func timeLimit() async throws {
     let test = try #require(Test.current)
     let snapshot = Test.Snapshot(snapshotting: test)
 
     #expect(snapshot.timeLimit == .seconds(60) * 999_999_999)
   }
+
+  /// Create a time limit trait representing the specified number of minutes, if
+  /// running on an OS which supports time limits.
+  ///
+  /// - Parameters:
+  ///   - minutes: The number of minutes the returned time limit trait should
+  ///     represent.
+  ///
+  /// - Returns: A time limit trait if the API is available, otherwise a
+  ///   disabled trait.
+  ///
+  /// This is provided in order to work around a bug where traits with
+  /// conditional API availability are not guarded by `@available` attributes on
+  /// `@Test` functions (rdar://127811571).
+  private static func _timeLimitIfAvailable(minutes: some BinaryInteger) -> any TestTrait {
+    guard #available(_clockAPI, *) else {
+      return .disabled(".timeLimit() not available")
+    }
+    return .timeLimit(.minutes(minutes))
+  }
 }
 
 extension Tag {

Diff for: Tests/TestingTests/Traits/TimeLimitTraitTests.swift

@@ -243,17 +243,21 @@ struct TimeLimitTraitTests {
 
 // MARK: - Fixtures
 
-func timeLimitIfAvailable(minutes: UInt64) -> any SuiteTrait {
+private func _timeLimitIfAvailable(minutes: UInt64) -> any SuiteTrait {
   // @available can't be applied to a suite type, so we can't mark the suite as
-  // available only on newer OSes.
+  // available only on newer OSes. In addition, there is a related, known bug
+  // where traits with conditional API availability are not guarded by
+  // `@available` attributes on their associated `@Test` function
+  // (rdar://127811571). That is not directly relevant here but is worth noting
+  // if this trait is ever applied to `@Test` functions in this file.
   if #available(_clockAPI, *) {
     .timeLimit(.minutes(minutes))
   } else {
     .disabled(".timeLimit() not available")
   }
 }
 
-@Suite(.hidden, timeLimitIfAvailable(minutes: 10))
+@Suite(.hidden, _timeLimitIfAvailable(minutes: 10))
 struct TestTypeThatTimesOut {
   @available(_clockAPI, *)
   @Test(.hidden, arguments: 0 ..< 10)