test-fd-util: skip test when lacking privileges to create a new namespace

To reproduce, as an unprivileged user start a docker container and build
and run the unit tests inside it:

$ docker run --rm -ti debian:bookworm bash
...
/* test_close_all_fds */
Successfully forked off '(caf-plain)' as PID 10496.
Skipping PR_SET_MM, as we don't have privileges.
(caf-plain) succeeded.
Failed to fork off '(caf-noproc)': Operation not permitted
Assertion 'r >= 0' failed at src/test/test-fd-util.c:392, function test_close_all_fds(). Aborting.

Partially fixes #35552

(cherry picked from commit 630a2e7)
(cherry picked from commit 5573ac7)
(cherry picked from commit b8c8556)
(cherry picked from commit 9818f55)
(cherry picked from commit dcdd4a6)
(cherry picked from commit c40e1d5)

Author: bluca

src/test/test-fd-util.c

@@ -347,6 +347,8 @@ TEST(close_all_fds) {
                         test_close_all_fds_inner();
                 _exit(EXIT_SUCCESS);
         }
+        if (ERRNO_IS_PRIVILEGE(r))
+                return (void) log_tests_skipped("Lacking privileges for test in namespace with /proc/ overmounted");
         assert_se(r >= 0);
 
         if (!is_seccomp_available()) {