Mock container execution to test failure modes

[cunha]

We need to mock the execution of the Scout container to test failure modes. Test cases to generate:

• Create $M$ containers that exit successfully after $N$ seconds ($M \approx 5$ and $N \approx 30$). Check that the polling thread handles completion and that containers are cleaned up.
• Create $M$ containers that never exit ($M \approx 5$), call shutdown after 30 seconds. Check that shutdown kills and removes containers.
• Create $M$ containers that terminate with an error after $N$ seconds ($M \approx 5$ and $N \approx 30$). Check that the polling thread handles errors correctly and that containers are cleaned up.