ADR for @ SnapshotTest

While we can infer this, we can have the scenario where people want some helper functions in their @SnapshotSuite which would then get marked as @SnapshotTest by default if they return valid views. We could have an opt-out mechanism instead which could be valid as helpers are generally used less than their tests that call them, but not quite as discoverable, especially for those used to SwiftTesting. The idea isn't less code always, especially when the mental model of SwiftTesting and tests in general is to opt 'in'.

Read the current @SnapshotSuite and @SnapshotTest macro behavior first, then compare the default marking behavior with the proposed opt-out approach. Done means an ADR records the chosen model, its rationale, and how helper functions are handled; the issue names no file or test to run.

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