test: stop using dup() needlessly

src/login/test-inhibit.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
 #include <unistd.h>
+#include <fcntl.h>
 
 #include "sd-bus.h"
 
@@ -30,7 +31,7 @@ static int inhibit(sd_bus *bus, const char *what) {
         assert_se(r >= 0);
         assert_se(fd >= 0);
 
-        return dup(fd);
+        return fcntl(fd, F_DUPFD_CLOEXEC, 3);
 }
 
 static void print_inhibitors(sd_bus *bus) {