build: give freebsd longer to run tests

The freebsd-386 and freebsd-amd64 builders are timing out sometimes.
This will give them some more breathing room.

Change-Id: Ib65bd172cca046a52861759a4232d7b4b6514fa8
Reviewed-on: https://go-review.googlesource.com/1994
Reviewed-by: Ian Lance Taylor <iant@golang.org>