[Impeller] Fix pipeline attachment layout in CanRenderToTexture. (#50413)

The pipeline descriptor defaults changed, so we need to align the attachments to the RenderPass in this test.
Should fix flutter/flutter#142868.

Author: bdero

impeller/renderer/renderer_unittests.cc

@@ -291,6 +291,8 @@ TEST_P(RendererTest, CanRenderToTexture) {
   auto pipeline_desc =
       BoxPipelineBuilder::MakeDefaultPipelineDescriptor(*context);
   pipeline_desc->SetSampleCount(SampleCount::kCount1);
+  pipeline_desc->ClearDepthAttachment();
+  pipeline_desc->SetStencilPixelFormat(PixelFormat::kS8UInt);
 
   ASSERT_TRUE(pipeline_desc.has_value());
   auto box_pipeline =