Remove agent tests

[axw]

The agent integration tests are of limited value. The tests are fairly basic, and we have since added fake APM Server implementations to the agent repos, which validate payloads.

The cost of maintaining the integration tests exceeds the value they are providing, therefore we should remove them. Later on we might replace them with something a little more maintainable. For example: disentangling stack/server provisioning, and distributing the tests to each agent repository. We can come back to that as needed.