MeshTools: reduce unwanted variability in the test.

It should fail just on the assert we expect, not on something else also.

src/Magnum/MeshTools/Test/CombineTest.cpp

@@ -171,7 +171,7 @@ void CombineTest::combineIndexedAttributesNotIndexed() {
         {}, indices, Trade::MeshIndexData{indices}, 1};
     Trade::MeshData b{MeshPrimitive::Lines,
         {}, indices, Trade::MeshIndexData{indices}, 1};
-    Trade::MeshData c{MeshPrimitive::Lines, 5};
+    Trade::MeshData c{MeshPrimitive::Lines, 1};
 
     std::ostringstream out;
     Error redirectError{&out};