selftests: netfilter: add conntrack stress test

Add a new test case to check:
 - conntrack_max limit is effective
 - conntrack_max limit cannot be exceeded from within a netns
 - resizing the hash table while packets are inflight works
 - removal of all conntrack rules disables conntrack in netns
 - conntrack tool dump (conntrack -L) returns expected number
   of (unique) entries
 - procfs interface - if available - has same number of entries
   as conntrack -L dump

Expected output with selftest framework:
 selftests: net/netfilter: conntrack_resize.sh
 PASS: got 1 connections: netns conntrack_max is pernet bound
 PASS: got 100 connections: netns conntrack_max is init_net bound
 PASS: dump in netns had same entry count (-C 1778, -L 1778, -p 1778, /proc 0)
 PASS: dump in netns had same entry count (-C 2000, -L 2000, -p 2000, /proc 0)
 PASS: test parallel conntrack dumps
 PASS: resize+flood
 PASS: got 0 connections: conntrack disabled
 PASS: got 1 connections: conntrack enabled
ok 1 selftests: net/netfilter: conntrack_resize.sh

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>