Get modal working + simplify ExecutionContext class

[justinchiu-cohere]

Changes

Modal

1. Creates and destructs modal.Sandbox within Modal.exec_with_timeout, allowing us to better control timeout.
2. Write report.json in Modal.exec_with_timeout
3. Issues only a single command to the sandbox: exec eval.sh + copy report.json to mount /vol
4. Return duration in exec

ExecutionContext

1. Remove helper functions that are not shared between Docker and Modal subclasses
2. Changed ExecutionContext.write_test_output to only write the test output to a log

Docker

1. Write report.json in Docker.exec_with_timeout
2. Remove unused helper functions

Remaining todos

1. Check for timeout in modal
2. Handle stdout/stderr the same way as docker

[wenting-zhao]

Changes

Modal

1. Creates and destructs modal.Sandbox within Modal.exec_with_timeout, allowing us to better control timeout.
2. Write report.json in Modal.exec_with_timeout
3. Issues only a single command to the sandbox: exec eval.sh + copy report.json to mount /vol
4. Return duration in exec

ExecutionContext

1. Remove helper functions that are not shared between Docker and Modal subclasses
2. Changed ExecutionContext.write_test_output to only write the test output to a log

Docker

1. Write report.json in Docker.exec_with_timeout
2. Remove unused helper functions

Remaining todos

1. Check for timeout in modal
2. Handle stdout/stderr the same way as docker

I handled the todos, they now worked.