Better testing story for Nexus Operations

[bergundy]

Currently the test environment support workflow run operations, sync operations get a client that panics for any method call.

To improve the testing story for Nexus, we'll want:

• Add a TestNexusEnvironment for users to test their operations outside of workflow context
  There are details to be ironed out here, e.g. how are sync operations going to work since they reference a Client.
• Add an OnNexusOperation method to the workflow test environment to integrate with current mocking and time skipping approach.