Adopt SuspendingClock.systemEpoch. (#1202)

This PR adopts the new `systemEpoch` API added with
[SE-0473](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0473-clock-epochs.md).

This API is back-deployed, so we don't need availability annotations on
Darwin. It is not available with the 6.1 toolchain though, so the old
`unsafeBitCast()` calls remain if building from source with a 6.1
toolchain.

### 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: grynspan

Sources/Testing/Events/TimeValue.swift

@@ -54,7 +54,11 @@ struct TimeValue: Sendable {
 
   @available(_clockAPI, *)
   init(_ instant: SuspendingClock.Instant) {
+#if compiler(>=6.2)
+    self.init(SuspendingClock().systemEpoch.duration(to: instant))
+#else
     self.init(unsafeBitCast(instant, to: Duration.self))
+#endif
   }
 }
 
@@ -110,7 +114,11 @@ extension Duration {
 @available(_clockAPI, *)
 extension SuspendingClock.Instant {
   init(_ timeValue: TimeValue) {
+#if compiler(>=6.2)
+    self = SuspendingClock().systemEpoch.advanced(by: Duration(timeValue))
+#else
     self = unsafeBitCast(Duration(timeValue), to: SuspendingClock.Instant.self)
+#endif
   }
 }