[REST API] Track assigned A/B test variant

[selanthiraiyan]

Part of: #8679

Description

This PR adds tracking for the REST API A/B test. This lets us track the variant assigned to the user.
Internal ref - pe5sF9-13h-p2

Changes

• Added track event to track the assigned variant.
• Unit test to validate tracking.
• Refactor unit tests.

Testing instructions

• Log out of the app
• Launch the app and skip onboarding if needed.
• From the prologue screen you can know which variant has been assigned to your device.
• Control will have the existing login screen has both WPCOM and store address login options.
• Treatment will only have the store address login option.
• Check Xcode logs and ensure that the following event is tracked.

Tracked rest_api_login_experiment, properties: [AnyHashable("experiment_variant"): "control"]

• Value for experiment_variant will vary based on the assigned variant. Currently, the experiment is staging, so we will always get the value control` here. 21039-explat-experiment

Screenshots

	Control	Treatment
Prologue screen

Tracks event registration

https://github.com/Automattic/tracks-events-registration/pull/1355

---

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

You can test the changes from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr8753-bc70ac9 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[pmusolino]

It could be just an unlucky case, but I always get control as an experiment, I tried multiple times (also erasing the content on the simulator) but I wasn't able to get the Treatment. 😢
What it could be? I imagine that on around 20 tests I should see a treatment at least.

[selanthiraiyan]

It could be just an unlucky case, but I always get control as an experiment, I tried multiple times (also erasing the content on the simulator) but I wasn't able to get the Treatment. 😢
What it could be? I imagine that on around 20 tests I should see a treatment at least.

I have configured 50% - 50% for control and treatment. I think we will always get the control variant until the experiment starts running. I have updated the PR description to include this detail.

[peril-woocommerce]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 2 days Please, make sure to get it merged by then or assign it to a later expiring milestone

Generated by 🚫 dangerJS

[selanthiraiyan]

@pmusolino

I have updated the abacus experiment to a newer version. (In the PR's base branch)

I made this change to address the feedback I received on the experiment proposal. pbxNRc-2i4-p2 I have validated this new experiment with my sandbox.

This is FYI, and this experiment version change won't affect this analytics tracking PR.

[pmusolino]

Go for it, it seems that unless the experiment is online we'll get always control