selftests/net: add variable NS_LIST for lib.sh

Add a global variable NS_LIST to store all the namespaces that setup_ns
created, so the caller could call cleanup_all_ns() instead of remember
all the netns names when using cleanup_ns().

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20231213060856.4030084-2-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>