Unify mockStorage objects in tests

[iconix]

PR #22 comment from @VishaalK:

We have 3-ish versions of mockStorage out there. I see one in tooltipHelper_tests.ts, another in sectionPicker_tests.ts. Can you add a low pri PBI to unify these?

[mttwc]

I'm wondering if we can get rid of all the mock implementations entirely and replace them with sinon mocks and have the same (or even better) level of testability.

[VishaalK]

^ I think this is the correct approach

[mttwc]

Resolved in #222 where we centralized the storage wiping in one spot for the frontEnd tests, and use MockStorage injection for extension backend tests.