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

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