Add make commands for tests

This adds the make commands required to build and run tests.

tests/cpp/test_main.cc

@@ -0,0 +1,8 @@
+// Copyright by Contributors
+#include <gtest/gtest.h>
+
+int main(int argc, char ** argv) {
+  testing::InitGoogleTest(&argc, argv);
+  testing::FLAGS_gtest_death_test_style = "threadsafe";
+  return RUN_ALL_TESTS();
+}