test(vsock_helper): close sockets

After we're done with a socket, close it, to avoid leaking resources.

We do not need to rebuild the rootfs for this, this specific binary is
compiled on the fly and SCP'd into the microVM.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

tests/host_tools/vsock_helper.c

@@ -84,7 +84,7 @@ int run_echo(uint32_t cid, uint32_t port) {
         }
     }
 
-    return 0;
+    return close(sock);
 }