More tests for mo_util_string (!63)

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## What is the new feature
Add more unit-tests for `mo_util_string`
## How is it implemented
Each public subroutine has its own test

## Mandatory steps before review
- [x] Gitlab CI passes _(Hint: use `make format` for linting)_ 
- [x] New feature is covered by additional unit tests
- [x] Mark the merge request as ready by removing `Draft:`

## Mandatory steps before merge
- [x] Test coverage does not decrease
- [x] Reviewed by a maintainer
- [x] Incorporate review suggestions
- [x] Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)

**You are not supposed to merge this request by yourself, the maintainers of fortan-support take care of this action!**

Co-authored-by: Jonas Jucker <jonas.jucker@c2sm.ethz.ch>
Approved-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Merged-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Changelog: other