Update backend unit tests coverage for kernelci_cache

Currently, the backend unit test coverage is around 40%.
The goal is to increase this coverage by adding tests across the entire backend codebase.

## General
- [ ] **checkouts.py** - current: 0%
- [ ] **utils.py** - current: 33%

## Queries
- [ ] **issues.py** - current: 20%
- [ ] **notifications.py** - current: 27%
- [ ] **tree.py** - current: 38%

# Related Issues
- #1494 