I tried to run the test with the trunk code, the tests are passed without any issue. What's the issue that you meet?
If you want to interceptSendToEndpoint, you need to make sure the uri string that you passed should be same with the endpoint.
The message "INTERCEPTED" is never printed and the request is not intercepted, nor skipped, it is still sent over internet and we see the content of "http://stackoverflow.com" through [httpclient.wire.content] messages.
Edit: And I am using global string to make sure there is no error in the URI name
Edit2: I updated the code in the first post so the test fails when the message is not intercepted, can you try it again ?