Increasing the timeout prevents the runtime test
to time out on the Plan 9 instances running on GCE.

Update golang/go#9491

Change-Id: Id9c2b0c4e59b103608565168655799b353afcd77
Reviewed-on: https://go-review.googlesource.com/2462Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>