Add unit tests for sandboxes

[xingyaoww]

What problem or use case are you trying to solve?

I found DockerSSHBox does not work well with multi-line commands, we should add unit tests to cover these cases.

Describe the UX of the solution you'd like

tests/unit/test_sandbox.py

Do you have thoughts on the technical implementation?

Adding tests is pretty straight forward, but we should better handle how to starts each unit test with the correct config (i.e., correct UID, correct workspace to mount to, etc).

Describe alternatives you've considered

Additional context

[rbren]

We have unit tests now! Thanks Xingyao!