Add test for agent resources:

@peterbourgon  @jmyounker @stuartnelson3 
Can you add basic block comments to each high-level "step" of this test? Like,

```go
// Set up an empty agent

// Create a container with some resource reservations

// Verify the HostResources are as we expect

// Destroy the container

// Verify the HostResources are reclaimed
```