Test the alertmanager routing config

Required for gitlab-com/gl-infra/scalability#214

At present, we don't run any tests on the alertmanager routing configure to ensure that it does what we expect it to. We should.

Changes to this configuration are not always obvious and may not do what you expect: see !2460 (merged) as an example.

Once we are more certain about this configuration doing what we think it should, we can start refactoring the routing rules without as much risk.

cc @AnthonySandoval