Insufficient integration tests for Knative components

[orfeas-k]

We should add more integration tests for Knative components, taking into account that knative-serving and knative-eventing charms should not just go to Active but also spin up deployments and pods in their corresponding namespaces. At the moment, we do not have sufficient tests for this and that's probably how we updated their version (PR #135) and ended up with knative-serving and eventing not being functional (issue #147).

[orfeas-k]

An update to this since we discussed a bit with @NohaIhab. Fact is that that we do have tests that deploy a KSVC and check that the KSVC response has 200 status. Should we have though tests that check that the deployments created in the corresponding namespaces (knative-eventing and knative-serving) during the charms' deployment complete? I 'm not sure if we have discussed this. If not, we can close this issue. But if yes, we should have a task for this.