126 Add unit tests

[aerickson14]

Progress on issue #126

What it Does

• Sets up CategoryList and its ViewModel to be testable
• Refactor DataStoreManager into a protocol so it can be mocked
• Added MockDataStoreManager so it can be replaced when testing view models that have it as a dependency
• Adds unit tests for CategoryListViewModel
• Turns on code coverage

How I Tested

Did a quick regression test of the app and everything still works as expected

Notes

I took some initiative on how to refactor the app so that it was more testable based on https://www.swiftbysundell.com/articles/writing-testable-code-when-using-swiftui/ Similarly I chose to use the given, when, then structure for unit tests. If you would like to see tests structured in a different way let me know.

Screenshot