Test: don't have unnecessarily large buffer size for XFB.

Caused me a few minutes figuring out what's wrong and why my overflow
query doesn't work.

src/Magnum/Test/PrimitiveQueryGLTest.cpp

@@ -280,7 +280,7 @@ void PrimitiveQueryGLTest::transformFeedbackPrimitivesWritten() {
     } shader;
 
     Buffer output;
-    output.setData({nullptr, 18*sizeof(Vector2)}, BufferUsage::StaticDraw);
+    output.setData({nullptr, 9*sizeof(Vector2)}, BufferUsage::StaticDraw);
 
     Mesh mesh;
     mesh.setPrimitive(MeshPrimitive::Triangles)