[ExPlat] Add A/A test to the app

[rachelmcr]

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We need to add the following A/A test (experiment) to the app: woocommerceios_explat_aa_test_202208

To add an ExPlat experiment to the app, add a new case to the ABTest enum in the Experiments framework.

The goal of this experiment is to see if there is any bias in our assignment implementation of ExPlat. Because this is an A/A experiment, there will be no differences between the control and treatment variations, so there's nothing more to be added aside from the test itself (to assign users to a control or treatment group).

Experiment design (internal ref): pbxNRc-1QS-p2

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