add sample hello_main.cc

single_file_tests/hello_main.cc

@@ -0,0 +1,7 @@
+#include <iostream>
+
+// compile: c++ --std=c++17 hello_main.cc
+auto main(int argc, char** argv) -> int {
+  std::cout << "hello, have: " << argc << " args" << std::endl;
+}
+