Create generalized mock / stub for storageManager

[robertrmartinez]

Type of issue

Test Enhancements

Description

When writing unit tests for modules which use storageManager, we need a better way to mock reading and setting cookies/local storage. As of now, lots of modules are doing it differently.

Relying on browsers in circle ci (and local browsers) to actually set and read back this data is not stable enough for unit tests.

So, we should create a mock which modules can import to their _spec files. Similar to how we mock XHR requests using test/mocks/server.js

See #5758 for the conversations and more details.

[stale]

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