If a test already isn't available, don't overwrite that error

util/test/demos/d3d11/d3d11_structured_buffer_misaligned_dirty.cpp

@@ -55,6 +55,9 @@ float4 main() : SV_Target0
   {
     D3D11GraphicsTest::Prepare(argc, argv);
 
+    if(!Avail.empty())
+      return;
+
     if(!opts.MapNoOverwriteOnDynamicBufferSRV)
       Avail = "Requires mappable buffer SRVs";
   }

util/test/demos/vk/vk_vertex_attr_zoo.cpp

@@ -274,6 +274,9 @@ void main()
 
     VulkanGraphicsTest::Prepare(argc, argv);
 
+    if(!Avail.empty())
+      return;
+
     if(physProperties.limits.maxVertexOutputComponents < 128)
       Avail = "Not enough vertex output components to run test";
   }