[test] Simplify shuffle_test while pleasing gcc static analysis

Author: serge-sans-paille

test/test_shuffle.cpp

@@ -621,12 +621,10 @@ struct shuffle_test
     {
         B b_lhs = B::load_unaligned(lhs.data());
         std::array<B, size> b_matrix;
-        for (size_t i = 0; i < size; ++i)
-            b_matrix[i] = b_lhs;
+        std::fill(b_matrix.begin(), b_matrix.end(), b_lhs);
         std::array<value_type, size * size> ref_matrix;
         for (size_t i = 0; i < size; ++i)
-            for (size_t j = 0; j < size; ++j)
-                ref_matrix[i * size + j] = lhs[i];
+          std::fill(&ref_matrix[i*size], &ref_matrix[(i + 1)*size], lhs[i]);
 
         INFO("transpose");
         xsimd::transpose(b_matrix.data(), b_matrix.data() + b_matrix.size());