selftests/bpf: Add existing connection bpf_*_ct_lookup() test

Add a test where we do a conntrack lookup on an existing connection.
This is nice because it's a more realistic test than artifically
creating a ct entry and looking it up afterwards.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/bpf/de5a617832f38f8b5631cc87e2a836da7c94d497.1660254747.git.dxu@dxuuu.xyz