Add a "test run" option

[msyriac]

Test runs would be different from dry runs. The latter is already implemented; it just checks for consistency and produces an output containing the commands that would be submitted.

A test run would also run each script with a special flag. Each script should enforce the following behavior: if the special flag is provided, then the script exits after imports and argument parsing. Perhaps an mbatch library wrapper could be implemented to allow this behavior to be quickly enforced in any script.

Test runs will prevent the batch from failing due to things like:

1. commands submitted from the wrong directory or with incorrect script paths
2. import errors
3. invalid arguments