Clarify that new SleepProvider methods are testing-only.

    /// on another thread.

    ///

    /// Call `release_advance` with the same reason string in order to unblock.

    ///

    /// This method is only for testing: it should never have any

    /// effect when invoked on non-testing runtimes.

    fn block_advance<T: Into<String>>(&self, _reason: T) {}

    /// Signify that the reason to withhold time advancing provided in a call to `block_advance` no

    /// longer exists, and it's fine to move time forward if nothing else is blocking advances.

    ///

    /// This method is only for testing: it should never have any

    /// effect when invoked on non-testing runtimes.

    fn release_advance<T: Into<String>>(&self, _reason: T) {}

    /// Allow a test running under mock time to advance time by the provided duration, even if the

    /// above `block_advance` API has been used.

    ///

    /// This method is only for testing: it should never have any

    /// effect when invoked on non-testing runtimes.

    fn allow_one_advance(&self, _dur: Duration) {}

}