DC tests don't validate certificates

In `delegated_credentials_test.go` `TestDCHandshakeServerAuth` sets `clientConfig.InsecureSkipVerify = true`. Because of how golang handles global state this is propagated to all later tests. This leads to test failures if the tests are run in a different order or individually.
This means that even if you put bit flips into the test certificate signatures the tests will still pass.

Start in delegated_credentials_test.go at TestDCHandshakeServerAuth and run the delegated-credentials tests individually and in different orders. Trace how clientConfig.InsecureSkipVerify affects later tests; done means certificate signature bit flips are rejected and test results do not depend on execution order.

Written by the indexing model from the issue text.