selftests/bpf: add missing \n to flow_dissector CHECK errors

Otherwise, in case of an error, everything gets mushed together.

Fixes: a5cb3346 ("selftests/bpf: make flow dissector tests more extensible")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>