Test Helpers should return the created resources

[Harwayne]

We probably should return the created resource for every create function, to make them consistent, but can be in a different PR.

Originally posted by @chizhg in #1995

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We have many functions in https://github.com/knative/eventing/blob/master/test/common/creation.go of the form CreateFooOrFail. Those functions should return the Foo that was created.

[chizhg]

/assign

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