Refactor API tests

This refactor adds tests for all supported OIDC types, and makes it
simpler to add new tests for new OIDC types.

* Add tests for K8s and GitHub OIDC types.
* Add additional verification for issued certificate values
* Add dedicated test for RootCert success, don't call RootCert in every test.
* Move common expectations to function. This provides a single place to
  check response values.
* Move common set up to dedicated functions.
* Lowercase all error messages, because style.

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>