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>