[ Widget Previews ] Add widget_preview_scaffold.shard to test the widget_preview_scaffold template contents (#166358)

Adds a new `widget_preview_scaffold.shard` directory which contains a
hydrated `widget_preview_scaffold` template. This will allow for us to
write widget tests against the widgets defined in the templates.

This PR doesn't add any widget tests and is only adding the ability to
run these tests in follow up changes.

Fixes https://github.com/flutter/flutter/issues/166416

.ci.yaml

@@ -1589,6 +1589,29 @@ targets:
       - engine/**
       - DEPS
 
+  - name: Linux tool_tests_widget_preview_scaffold
+    recipe: flutter/flutter_drone
+    timeout: 60
+    bringup: true
+    properties:
+      add_recipes_cq: "true"
+      dependencies: >-
+        [
+          {"dependency": "android_sdk", "version": "version:35v1"},
+          {"dependency": "open_jdk", "version": "version:21"}
+        ]
+      shard: tool_tests
+      subshard: widget_preview_scaffold
+      tags: >
+        ["framework", "hostonly", "shard", "linux"]
+    runIf:
+      - dev/**
+      - packages/flutter_tools/**
+      - bin/**
+      - .ci.yaml
+      - engine/**
+      - DEPS
+
   - name: Linux_android_emu android_engine_vulkan_tests
     recipe: flutter/flutter_drone
     timeout: 60