selftests/bpf: Use reno instead of dctcp

Andrey pointed out that we can use reno instead of dctcp for CC
tests and drop CONFIG_TCP_CONG_DCTCP=y requirement.

Fixes: beecf11b ("bpf: Bpf_{g,s}etsockopt for struct bpf_sock_addr")
Suggested-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200501224320.28441-1-sdf@google.com