infra: fix silkworm_infra_test_util build on macOS (#1307)

silkworm/infra/test_util/dummy.cpp

@@ -0,0 +1,17 @@
+/*
+   Copyright 2023 The Silkworm Authors
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+// Empty compilation unit just to make silkworm_infra_test_util build under macOS