Improve unit tests for mo_util_sort (!79)

## What is the bug
Fix a bug in unit test ([here](!79 (diffs))

) and improve code coverage
## How do you fix it
The code coverage is improved to 100% for `mo_util_sort`, and new tests with random values are added.

Approved-by: Jonas Jucker <jonas.jucker@env.ethz.ch>
Merged-by: Jonas Jucker <jonas.jucker@env.ethz.ch>
Changelog: bugfix