context: add test for WithDeadline in the past

Adds a test case for calling context.WithDeadline() where the deadline
exists in the past.  This change increases the code coverage of the
context package.

Change-Id: Ib486bf6157e779fafd9dab2b7364cdb5a06be36e
Reviewed-on: https://go-review.googlesource.com/25007


Reviewed-by: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Sameer Ajmani <sameer@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>