message: add test for copy_val_to_host
diff: @@ -218,6 +218,18 @@ TEST(ReferenceExecutor, CopiesData)
  }
  +TEST(ReferenceExecutor, CopiesSingleValue)
  +{
  + exec_ptr ref = gko::ReferenceExecutor::create();
  + int *el = ref->alloc<int>(1);
  + el[0] = 83683;
  +
  + EXPECT_EQ(83683, ref->copy_val_to_host(el));
  +
  + ref->free(el);
  +}
  +
  +
  TEST(ReferenceExecutor, CopiesDataFromOmp)
  {
  int orig[] = {3, 8};
  