Set up basic e2e test that deploys the webhook and ensures we can POST a challenge

[munnerz]

This test probably doesn't need cert-manager itself running for the time being.

I think the test should cover:

• Building this project
• Deploying the Helm chart
• Ensuring that the pod starts
• Ensuring that the APIService is marked Ready
• Ensuring that we can POST a ChallengePayload resource and get a 200 response

In future we may want to explore having some form of extended e2e that actually deploys and runs a version of cert-manager too, but let's keep the scope small for now 😄