selftests/clone3: test clone3 with CLONE_NEWTIME

Verify that clone3 can be called successfully with CLONE_NEWTIME in
flags.

Cc: Andrey Vagin <avagin@openvz.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>