tests: add CLONE_NEWTIME setns tests

Now that pidfds support CLONE_NEWTIME as well enable testing them in the
setns() testuite.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Dmitry Safonov <dima@arista.com>
Cc: Andrei Vagin <avagin@gmail.com>
Link: https://lore.kernel.org/r/20200706154912.3248030-5-christian.brauner@ubuntu.com