Allow to disable google-test completely

It will be useful for boxes without internet at all, and when people
don't need to run our tests

Change-Id: I85f2d9b3760ee6edbbd44318fe0893f39817b8e9
Reviewed-on: http://review.couchbase.org/24301
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>