[Woo POS] Improve unit test coverage

[backwardstruck]

We should try to do this for all the new POS classes.

Discussed here: https://docs.google.com/document/d/10JKyruY8xHMMTV3RRGlPkVeTh-wiVMtzSaN-lgIN6DU/edit?disco=AAABQsTmXAA

### Tasks
- [ ] https://github.com/woocommerce/woocommerce-android/issues/11928
- [ ] common
- [ ] https://github.com/woocommerce/woocommerce-android/issues/11932
- [ ] root
- [ ] util

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.