New contract tests don't pass against stage or prod servers

[mythmon]

The new contract tests work in CI (sometimes, see #1826), but don't pass against prod and stage. Looking at the problems in the test failures, it is definitely a shortcoming of the tests, not that the servers are currently broken.

This will prevent deploying the service, and is causing trouble with local development for me. I also suspect this is related to recent CI failures.

I'm going to revert the tests to their prior state, since this is now blocking our development. @chartjes, can you update the tests so that they are more reliable, and pass against our real deployments? After that we can re-land these tests.