tst-ttyname: Fix namespace setup for Fedora

On Fedora, the previous initialization sequence did not work and
resulted in failures like:

info:  entering chroot 1
info:    testcase: basic smoketest
info:      ttyname: PASS {name="/dev/pts/5", errno=0}
info:      ttyname_r: PASS {name="/dev/pts/5", ret=0, errno=0}
error: ../sysdeps/unix/sysv/linux/tst-ttyname.c:122: write (setroups, "deny"): Operation not permitted
info:  entering chroot 2
error: ../sysdeps/unix/sysv/linux/tst-ttyname.c:122: write (setroups, "deny"): Operation not permitted
error: 2 test failures

Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
(cherry picked from commit 8db7f48c)