Add code coverage (!57)

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## What is the new feature
Add code coverage.
## How is it implemented
Directly add in pipeline. The total coverage percentage will be shown in the MRs.  
Reference: [coverage](https://docs.gitlab.com/ee/ci/testing/code_coverage.html), [coverage visualization](https://docs.gitlab.com/ee/ci/testing/test_coverage_visualization.html

)
- [x] Some CMake/FortUTF files are picked up in the coverage. Find a way to remove them.
- [x] Backtrace test fails in sphinx runner.
- [x] Ragel generated files cannot be processed by gcovr.  
* Backtrace function and codes generated by Ragel are not supported in this version.

## 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] Reviewed by a maintainer
- [ ] Incorporate review suggestions
- [ ] 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!**

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