Introduce 'test' target, add /proc/net/tcp parsing to it

The test itself doesn't do much yet, but might shed some light on what's going
on in https://github.com/raboof/nethogs/issues/22

conninode_test.cpp

@@ -0,0 +1,28 @@
+#include "conninode.cpp"
+
+local_addr *local_addrs = NULL;
+
+int main() {
+  if (!addprocinfo("testfiles/proc_net_tcp")) {
+    std::cerr << "Failed to load testfiles/proc_net_tcp" << std::endl;
+    return 1;
+  }
+  if (!addprocinfo("testfiles/proc_net_tcp_big")) {
+    std::cerr << "Failed to load testfiles/proc_net_tcp_big" << std::endl;
+    return 2;
+  }
+
+#if defined(__APPLE__)
+  if (!addprocinfo("net.inet.tcp.pcblist")) {
+    std::cerr << "Failed to load net.inet.tcp.pcblist" << std::endl;
+    return 3;
+  }
+#else
+  if (!addprocinfo("/proc/net/tcp")) {
+    std::cerr << "Failed to load /proc/net/tcp" << std::endl;
+    return 4;
+  }
+#endif
+
+  return 0;
+}