test: fix vsock helper hanging after exit

When running with an Ubuntu 22.04 rootfs, vsock_helper can hang and
never exit.

Fixed it by also closing STDERR when daemonizing.

Signed-off-by: Pablo Barbáchano <pablob@amazon.com>

Author: pb8o

tests/host_tools/vsock_helper.c

@@ -180,6 +180,7 @@ int main(int argc, char **argv) {
             setsid();
             close(STDIN_FILENO);
             close(STDOUT_FILENO);
+            close(STDERR_FILENO);
         }
         else {
             port = atoi(argv[2]);