Add webhook secret to test config
Add webhook secret to test config

mythmon/taskcluster-github,mrrrgn/taskcluster-github,taskcluster/taskcluster-github,rail/taskcluster-github,petemoore/taskcluster-github,mrrrgn/taskcluster-github,mrrrgn/taskcluster-github-1,mythmon/taskcluster-github,rail/taskcluster-github,mrrrgn/taskcluster-github-1,petemoore/taskcluster-github