selftests/net: reduce xfrm_policy test time

The check_random_order test add/get plenty of xfrm rules, which consume
a lot time on debug kernel and always TIMEOUT. Let's reduce the test
loop and see if it works.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20240514095227.2597730-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>