selftests/bpf: Fix flow dissector tests

- update custom loader to search by name, not section name
- update bpftool commands to use proper pin path

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211021214814.1236114-4-sdf@google.com