selftests/bpf: Extend test_flow_dissector to cover link creation

Extend the existing flow_dissector test case to run tests once using direct
prog attachments, and then for the second time using indirect attachment
via link.

The intention is to exercises the newly added high-level API for attaching
programs to network namespace with links (bpf_program__attach_netns).

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200531082846.2117903-13-jakub@cloudflare.com