Unit testing for slurm_cli_pre_submit function

[halfflat]

We currently have unit tests for various helper functions, but not for the main function that ultimately does the work.

For tracking and mocking the read and set options, it would be helpful to make our own translator to and from the 'fake' options table backed by slurm which does not support e.g. iteration. Changes to the options table could also then be more easily reported in debug output from the cli_filter itself.

[halfflat]

Partly resolved in #13 but more to do.