[Paywalls] Add Emerge Snapshot Tests

[MarkVillacampa]

This PR:

• Adds a test target to PaywallTester.
• Adds Emerge's SnapshotPreviews package to the test target.
• Adds Emerge's Fastlane plugin and a new lane which uploads PaywallsTester to Emerge.
• Adds a CircleCI job to run the lane.

Shoutout to @noahsmartin for all the help!

[joshdholtz]

This looks so good! Just a few small naming suggestions 😊

But also, is there a reason to not run this on every PR? I mean, not every commit is doing a UI change but we do run it on every PR for Android (see here) and its kind of nice, IMO 😇

Or if we can't do that... it might be nice to have another command that is simlar to the "test all" one but only for snapshots? 🤷‍♂️ Thoughts?