Add e2e test case for Sequence

**Problem**
A new Pipeline/Sequence CRD has been added. We need e2e test coverage for it.
https://github.com/knative/eventing/pull/1239
https://github.com/knative/eventing/issues/1376

**Exit Criteria**
e2e test case(s) are added and can consistently pass.

/assign @Fredy-Z 