test(container): render Cell slices test against .zon input

src/testing.zig

@@ -71,10 +71,50 @@ pub const Renderer = struct {
 
         for (container.elements.items) |*element| try this.render(T, element);
     }
+
+    pub fn save(this: @This(), writer: anytype) !void {
+        try std.zon.stringify.serialize(this.screen, .{ .whitespace = false }, writer);
+    }
 };
 
 /// This function is intended to be used only in tests. Test if a `Container`'s
 /// rendered contents are equal to the expected `Cell` slice.
+///
+/// # Test data creation
+///
+/// Create a .zon file containing the expected `Cell` slice using the `zterm.testing.Renderer.save` method:
+///
+/// ```zig
+/// const file = try std.fs.cwd().createFile("test/container/border/all.zon", .{ .truncate = true });
+/// defer file.close();
+///
+/// const allocator = std.testing.allocator;
+/// var renderer: testing.Renderer = .init(allocator, size);
+/// defer renderer.deinit();
+///
+/// try container.handle(.{ .resize = size });
+/// try renderer.render(Container(event.SystemEvent), &container);
+/// try renderer.save(file.writer());
+/// ```
+///
+/// # Testing against created data
+///
+/// Then later load that .zon file at compile time and run your test against this `Cell` slice.
+///
+/// ```zig
+/// var container: Container(event.SystemEvent) = try .init(std.testing.allocator, .{
+///     .border = .{
+///         .color = .green,
+///         .sides = .all,
+///     },
+/// }, .{});
+/// defer container.deinit();
+///
+/// try testing.expectContainerScreen(.{
+///     .rows = 20,
+///     .cols = 30,
+/// }, &container, @import("test/container/border.all.zon"));
+/// ```
 pub fn expectContainerScreen(size: Size, container: *Container(event.SystemEvent), expected: []const Cell) !void {
     const allocator = std.testing.allocator;
     var renderer: Renderer = .init(allocator, size);