The test-util feature flag is only intended to be used with tests.
However, it is possible to enable it in release mode accidentally. This
patch reduces the overhead of `Instant::now()` when the `test-util`
feature flag is enabled but `time::pause()` is not called.
The optimization is implemented by adding a static atomic flag that
tracks if `time::pause()` has ever been called. In `Instant::now()`, the
atomic flag is first checked before the thread-local and mutex are
accessed.
