Existing tests don't check overflow and underflow case for subtraction
monotonic time.

Updates #17858

Change-Id: I95311440134c92eadd7d5e409a0fc7c689e9bf41
Reviewed-on: https://go-review.googlesource.com/107056
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>