gh-100005: Skip test_script_as_dev_fd() on FreeBSD (GH-100006)

On FreeBSD, skip test_script_as_dev_fd() of test_cmd_line_script if
fdescfs is not mounted (at /dev/fd).
(cherry picked from commit 038b151963d9d4a5f4c852544fb5b0402ffcb218)

Co-authored-by: Victor Stinner <vstinner@python.org>