selftests/bpf: Add tests for new nf_conntrack kfuncs

Introduce selftests for the following kfunc helpers:
- bpf_xdp_ct_alloc
- bpf_skb_ct_alloc
- bpf_ct_insert_entry
- bpf_ct_set_timeout
- bpf_ct_change_timeout
- bpf_ct_set_status
- bpf_ct_change_status

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220721134245.2450-12-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>