sandbox: restrict run time
sandbox: restrict run time

Assuming that wall time is a reasonable proxy for cpu time.

Change-Id: If6a3fe141f16d4b400047154d1513c6d32c945c5
Reviewed-on: https://go-review.googlesource.com/2851
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

neollie/playground,golang/playground,phoorichet/playground,phoorichet/playground,phoorichet/playground,golang/playground,adams-sarah/playground,heshed/playground,neollie/playground,gophergala2016/playground-plus,heshed/playground,adams-sarah/playground,adams-sarah/playground,gophergala2016/playground-plus,heshed/playground,gophergala2016/playground-plus,neollie/playground,golang/playground