web-app: Add e2e tests

[kimwnasptd]

/kind feature

Describe the solution you'd like
We should add e2e tests to ensure the web app is compatible with the changes in the InferenceService CRD. I believe these could be some good first e2e tests to start:

• Test that the app can be deployed. We can just test the standalone mode and not expect any other Kubeflow components.
• Test that the backend's route for creating InferenceServices work as expected. We can have a list of different CRs to test with.
• Test that the backend's route for deleting InferenceServices work as expected.
• Test that the backend's route for exposing the list of existing InferenceServices works as expected
• Test that the user can delete an InferenceService via the UI
• Test that the user can create an InferenceService via the UI

These should be fairly trivial to implement and could give us a base coverage. I'm also expecting that the test runner will have access to a cluster with KFServing CRDs installed. Might need to add a step to create this cluster but we can iron out the technical details in this issue.

Anything else you would like to add:
I'd like to start working on this as soon as the manifests of the app are merged kserve/kserve#1505 and the first iteration of the code is in kserve/kserve#1328.

I have one question regarding how to add these tests. Are you using the Optional-test infra to run integration and e2e tests using the prow_config.yaml file?

cc @kubeflow/wg-serving-leads @PatrickXYS @theofpa
/assign @kimwnasptd

[PatrickXYS]

I have one question regarding how to add these tests. Are you using the Optional-test infra to run integration and e2e tests using the prow_config.yaml file?

Short answer is yes, this is where we define kserve/kserve test in optional-test-infra. https://github.com/kubeflow/testing/blob/master/aws/User/clusters/optional-test-infra-prow/namespaces/prow/configmap/config.yaml#L121-L133