Add some more common test componets in pkg/test/

[srinivashegde86]

This PR adds:

1. Export GetConfigMap method
2. Create methods for working with k8s pods
3. Create a new file crd.go that adds few methods to deal with crd's.
4. Add DOCKER_REPO and TAG flags here as we need it in other repos.
5. Add a few wait cheks for k8s pods

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[srinivashegde86]

/cc @bbrowning

[srinivashegde86]

knative/eventing-contrib#178 was adding a few k8s specific details to the eventing-sources repo. These should be in the pkg/test repo instead as these are generic to k8s and can be re-used by others

[srinivashegde86]

Part of #26

[jessiezcc]

/lgtm