[Impeller] Fix validation errors in RendererTest. (#46076)

Vulkan validation was tripping on the fact that renderer tests were rendering to the root render pass. This root render pass doesn't contain a stencil buffer. However, `MakeDefaultPipelineDescriptor` assumes a stencil and color attachment. The other backends are resilient to this mismatch since there is no compat render pass created upfront. But Vulkan was sad. This should only happen in the low level tests. In the higher levels of the framework, we have variants that make sure there is a pipeline pass and render pass compatibility.

Fixes validations of the kind:

```
--- Vulkan Debug Report  ----------------------------------------
|                Severity: Error
|                    Type: { Validation }
|                 ID Name: VUID-vkCmdDraw-renderPass-02684
|               ID Number: 1349015333
|       Queue Breadcrumbs: [NONE]
|  CMD Buffer Breadcrumbs: [NONE]
|         Related Objects: RenderPass [16305153808034431137] [Playground Render Pass], RenderPass [18100546345029861533] [Compat Render Pass: BoxFade Pipeline]
|                 Trigger: Validation Error: [ VUID-vkCmdDraw-renderPass-02684 ] Object 0: handle = 0xe2478b00000000a1, name = Playground Render Pass, type = VK_OBJECT_TYPE_RENDER_PASS; Object 1: handle = 0xfb320f000000009d, name = Compat Render Pass: BoxFade Pipeline, type = VK_OBJECT_TYPE_RENDER_PASS; | MessageID = 0x50685725 | vkCmdDraw: RenderPasses incompatible between active render pass w/ VkRenderPass 0xe2478b00000000a1[Playground Render Pass] and pipeline state object w/ VkRenderPass 0xfb320f000000009d[Compat Render Pass: BoxFade Pipeline] Attachment 4294967295 is not compatible with 1: The first is unused while the second is not.. The Vulkan spec states: The current render pass must be compatible with the renderPass member of the VkGraphicsPipelineCreateInfo structure specified when creating the VkPipeline bound to VK_PIPELINE_BIND_POINT_GRAPHICS (https://vulkan.lunarg.com/doc/view/1.3.224.1/mac/1.3-extensions/vkspec.html#VUID-vkCmdDraw-renderPass-02684)
-----------------------------------------------------------------

```