Make throttle test a bit more reliable

The throttle tests fail a lot on the build bot. With increasing the
timeout, the test is more reliable (at least locally).

Change-Id: I6090cfb34ba0a3d158eaa7e3f0dcd49dd2502daf
Reviewed-on: http://review.couchbase.org/37176
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: buildbot <build@couchbase.com>