Make boot_time::timeout() timeout immediately when passing zero duration

It's possible that the duration returned by Quiche timeout()
is Duration::ZERO. However, boot_time::sleep(Duration::ZERO).await
doesn't wake up immediately in tokio::select. This results in
missing one Quiche on_timeout() call.

Bug: 223356608
Test: cd packages/modules/DnsResolver && atest
Change-Id: Ib9d7fc9792158262d87a86f4e474e618efdbf408