Add tests to data-store

[ryanlanciaux]

Add tests for the reducers.

• [x] Data Reducer
• [x] Griddle Reducer
• [x] Local Reducer
• [ ] Position Reducer (@joellanciaux)
• [x] Selection Reducer
• [x] Subgrid Reducer

[ryanlanciaux]

Position reducer has been moved to a separate project. This should be tested but the issue should belong in that project.