Reduce scope of mock patch to prevent crosstalk

[desertaxle]

Closes #13963

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[chrisguidry]

I don't think this is materially different? When you patch a class's instance method, you're going to end up doing it for all instances process-wide

[zhen0]

@desertaxle - does this need a re-review or should it be closed?

[desertaxle]

Fixing this test will require a different approach