Add cat test case

test/test_cat.cc

@@ -0,0 +1,15 @@
+#include <iostream>
+#include "../subprocess.hpp"
+
+using namespace subprocess;
+
+int main() {
+  auto p = Popen({"cat", "-"},
+		 input{PIPE},
+		 output{PIPE});
+  const char* msg = "through stdin to stdout";
+  auto res = p.communicate(msg, strlen(msg)).first;
+  std::cout << res.buf.data() << std::endl;
+
+  return 0;
+}