Use defineProperty on global in setupTests.js

[tannerkrewson]

I tried to use this code example to mock the localStorage in a unit test. I was getting a "not a mock error", so I eventually ended up doing a console.log(localStorageMock == global.localStorage) directly after the line I removed in this PR, and it would return false. With this fix, it returns true and my tests are working.

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[ianschmitz]

Closing as a dupe of #6882. Thanks for the PR!