Tests: Add a test for concurrent use of std430 and push_constant.

Test/vulkan.frag

@@ -43,7 +43,7 @@ layout(push_constant) buffer pcb {            // ERROR, not on a buffer
 layout(push_constant) uniform float pcfloat;  // ERROR 2X: not on a non-block, and non-opaque outside block
 
 layout(push_constant) uniform;                // ERROR, needs an object
-
+layout(std430, push_constant) uniform pcb1 { int a; } pcb1inst;
 layout(push_constant) uniform pcb2 {
     int a;
 };                                            // Okay now to have no instance name