Test conversion follow-up

[jaswilli]

Todos in follow up to #315.

Planned tests to add:

• Check loading GCP ID from file with the local_endpoint module
  • Make sure this covers testing the windows variant (previously broken, Fix linting errors caught by new pycodestyle #321)
• Add unicode to a few response objects, make sure it comes through translations okay
• Full successful login flow
• Functional tests for RenewingAuthorizer usage
  • RefreshToken and ClientCredentials
  • Check 401s on first response and second response
  • Check with a client method with retry_401s=False
  • RenewingAuthorizer callout may fail (e.g. network error)
  • Ensure on_refresh callback is invoked
  • Propagate errors from malformed on_refresh callback
    • Does the Authorizer state change? Documenting this behavior would be a good add

[sirosen]

We should also look at #151 again. Those "untestable" things were one of the drivers of the conversion.

[sirosen]

I'm copying from #151 as a checkbox list, and I've closed it to consolidate. Some of these are not relevant anymore, some are.

• Myproxy activation. Requires a stable endpoint with a myproxy login for SDK Tester. (may be deprecated at some point anyways)
• Login-flow. Requires automating a browser and SDK Tester's login credentials being made public.
• Code exchange. Requires login-flow or a non expiring test code.
• Live openid JWT decoding. Requires login-flow, a non expiring id_token, or an auth resource other than code exchange that responds with an an id_token.
• Dependent token grants. Requires a resource server client and scopes to be set up.
• Managed endpoint resources. Requires SDK Tester to have privileges on a managed endpoint, which would give public access to privileges on a managed endpoint.

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At a glance, I think we should focus on whatever gives us more code/conditional coverage, and that makes the last two -- managed endpoint tests and dependent token tests -- the most attractive. They're straightforward and "just need doing".
I'm not sure a login flow, JWT handling, code exchange, or activation are particularly useful to test. (Though some tests of the activation response might be wise, if we don't have those.)