selftests: netfilter: run conntrack_dump_flush in netns

selftests: netfilter: run conntrack_dump_flush in netns

This test will fail if the initial namespace has conntrack
active due to unexpected number of flows returned on dump:

  conntrack_dump_flush.c:451:test_flush_by_zone:Expected ret (7) == 2 (2)
  test_flush_by_zone: Test failed
  FAIL  conntrack_dump_flush.test_flush_by_zone
  not ok 2 conntrack_dump_flush.test_flush_by_zone

Add a wrapper that unshares this program to avoid this problem.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://patch.msgid.link/20241104142529.2352-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>