Update `PreviewTests` stencil to use `displayName` if provided.

[markst]

If I wrap my previews in the following:

public struct UIElementPreview<Value: View>: View {

    private let dynamicTypeSizes: [ContentSizeCategory] = [.extraSmall, .large, .extraExtraExtraLarge]

    /// Filter out "base" to prevent a duplicate preview.
    private let localizations = Bundle.main.localizations.map(Locale.init).filter { $0.identifier != "base" }

    private let viewToPreview: Value

    public init(_ viewToPreview: Value) {
        self.viewToPreview = viewToPreview
    }

    public var body: some View {
        Group {
            ForEach(localizations, id: \.identifier) { locale in
                self.viewToPreview
                    .previewLayout(PreviewLayout.sizeThatFits)
                    .padding()
                    .environment(\.locale, locale)
                    .previewDisplayName(Locale.current.localizedString(forIdentifier: locale.identifier))
            }

            self.viewToPreview
                .previewLayout(PreviewLayout.sizeThatFits)
                .padding()
                .background(Color(.systemBackground))
                .environment(\.colorScheme, .dark)
                .previewDisplayName("Dark Mode")

            ForEach(dynamicTypeSizes, id: \.self) { sizeCategory in
                self.viewToPreview
                    .previewLayout(PreviewLayout.sizeThatFits)
                    .padding()
                    .environment(\.sizeCategory, sizeCategory)
                    .previewDisplayName("\(sizeCategory)")
            }

        }
    }
}

My generated snapshots have more context rather than just incremental numbers.

test_prefireView_Preview.PrefireView.png

Closes #16