Add integration test for Pinniped Supervisor/JWTAuthenticator end-to-end flow

[ankeesler]

Is your feature request related to a problem? Please describe.

We don't currently have an integration test that tests the end-to-end flow of 1) obtaining a token set from the Pinniped Supervisor and 2) using that token set to authenticate to a cluster via a JWTAuthenticator. We should add this as an integration test, since it is quite possible the most common Pinniped architecture that folks will deploy.

Describe the solution you'd like

We should have an integration test that 1) logs into the Pinniped Supervisor and gets a token set issued for a particular cluster and 2) uses that token set to authenticate to the particular cluster via a JWTAuthenticator.

Describe alternatives you've considered

None.

Are you considering submitting a PR for this feature?

• How will this project improvement be tested?
  It is a test, so N/A.
• How does this change the current architecture?
  It is a test, so N/A.
• How will this change be backwards compatible?
  It is a test, so N/A.
• How will this feature be documented?
  It is a test, so N/A.

Additional context

This came up during development of #258. We punted on this to try to optimize for the shortest path to a working system.

[ankeesler]

I think @mattmoyer did this in #276 specifically in 0b38d6c.

Closing.