Fix BarycentricsTest test (#3476)

The render target was unnecessary huge causing failures on Microsoft Basic Render Driver
(which supports Barycentrics in newer Windows versions).

tools/clang/test/HLSL/ShaderOpArith.xml

@@ -1028,7 +1028,7 @@
         { {   1.0f, -1.0f , 0.0f }, { 0.0f, 1.0f, 0.0f, 1.0f } },
         { {  -1.0f, -1.0f , 0.0f }, { 0.0f, 0.0f, 1.0f, 1.0f } }
       </Resource>
-    <Resource Name="RTarget" Dimension="TEXTURE2D" Width="5120" Height="9600" Format="R32G32B32A32_FLOAT" Flags="ALLOW_RENDER_TARGET" InitialResourceState="COPY_DEST" ReadBack="true" />
+    <Resource Name="RTarget" Dimension="TEXTURE2D" Width="1280" Height="2400" Format="R32G32B32A32_FLOAT" Flags="ALLOW_RENDER_TARGET" InitialResourceState="COPY_DEST" ReadBack="true" />
     <DescriptorHeap Name="RtvHeap" NumDescriptors="1" Type="RTV">
       <Descriptor Name="RTarget" Kind="RTV"/>
     </DescriptorHeap>